diff --git a/crates/tranquil-comms/src/email/message.rs b/crates/tranquil-comms/src/email/message.rs
new file mode 100644
index 0000000..7d5df8f
--- /dev/null
+++ b/crates/tranquil-comms/src/email/message.rs
@@ -0,0 +1,151 @@
+use lettre::Message;
+use lettre::message::Mailbox;
+use lettre::message::header::ContentType;
+use uuid::Uuid;
+
+use super::types::EmailDomain;
+use crate::sender::SendError;
+use crate::types::QueuedComms;
+
+pub(super) fn build(from: &Mailbox, qc: &QueuedComms) -> Result<Message, SendError> {
+    let to: Mailbox = qc
+        .recipient
+        .parse()
+        .map_err(|e: lettre::address::AddressError| SendError::InvalidRecipient(e.to_string()))?;
+    let subject = qc.subject.as_deref().unwrap_or("Notification");
+    let message_id = format!("<{}@{}>", Uuid::new_v4(), from.email.domain());
+    Message::builder()
+        .from(from.clone())
+        .to(to)
+        .subject(subject)
+        .message_id(Some(message_id))
+        .header(ContentType::TEXT_PLAIN)
+        .body(qc.body.clone())
+        .map_err(|e| SendError::MessageBuild(e.to_string()))
+}
+
+pub(super) fn recipient_domain(message: &Message) -> Result<EmailDomain, SendError> {
+    let envelope = message.envelope();
+    let first = envelope
+        .to()
+        .first()
+        .ok_or_else(|| SendError::MessageBuild("envelope has no recipients".to_string()))?;
+    EmailDomain::parse(first.domain())
+        .map_err(|e| SendError::InvalidRecipient(format!("invalid recipient domain: {e}")))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::types::{CommsChannel, CommsStatus, CommsType};
+    use chrono::Utc;
+    use uuid::Uuid;
+
+    fn from_mailbox() -> Mailbox {
+        "Test Sender <noreply@nel.pet>".parse().unwrap()
+    }
+
+    fn fixture(recipient: &str, subject: Option<&str>, body: &str) -> QueuedComms {
+        QueuedComms {
+            id: Uuid::new_v4(),
+            user_id: None,
+            channel: CommsChannel::Email,
+            comms_type: CommsType::Welcome,
+            status: CommsStatus::Pending,
+            recipient: recipient.to_string(),
+            subject: subject.map(String::from),
+            body: body.to_string(),
+            metadata: None,
+            attempts: 0,
+            max_attempts: 3,
+            last_error: None,
+            created_at: Utc::now(),
+            updated_at: Utc::now(),
+            scheduled_for: Utc::now(),
+            processed_at: None,
+        }
+    }
+
+    #[test]
+    fn build_basic_message() {
+        let msg = build(
+            &from_mailbox(),
+            &fixture("user@nel.pet", Some("Welcome"), "Hello world."),
+        )
+        .unwrap();
+        let raw = String::from_utf8(msg.formatted()).unwrap();
+        let lower = raw.to_lowercase();
+        assert!(raw.contains("From: \"Test Sender\" <noreply@nel.pet>"));
+        assert!(raw.contains("To: user@nel.pet"));
+        assert!(raw.contains("Subject: Welcome"));
+        assert!(lower.contains("content-type: text/plain"));
+        assert!(raw.contains("Hello world."));
+    }
+
+    #[test]
+    fn utf8_subject_is_encoded() {
+        let msg = build(
+            &from_mailbox(),
+            &fixture("user@nel.pet", Some("héllo wörld"), "Body"),
+        )
+        .unwrap();
+        let raw = String::from_utf8(msg.formatted()).unwrap();
+        assert!(raw.contains("=?utf-8?"));
+        assert!(!raw.contains("héllo"));
+    }
+
+    #[test]
+    fn header_injection_rejected() {
+        let result = build(
+            &from_mailbox(),
+            &fixture("x@nel.pet\r\nBcc: evil@x", Some("s"), "b"),
+        );
+        assert!(matches!(result, Err(SendError::InvalidRecipient(_))));
+    }
+
+    #[test]
+    fn subject_crlf_does_not_inject_headers() {
+        let msg = build(
+            &from_mailbox(),
+            &fixture("user@nel.pet", Some("hi\r\nBcc: evil@nel.pet"), "body"),
+        )
+        .expect("subject CRLF should be encoded, not rejected");
+        let raw = String::from_utf8(msg.formatted()).unwrap();
+        assert!(
+            !raw.contains("Bcc:"),
+            "CRLF in subject must not produce a Bcc header: {raw}"
+        );
+        assert!(
+            raw.contains("Subject: ="),
+            "subject with non-printable chars should be RFC 2047 encoded: {raw}"
+        );
+    }
+
+    #[test]
+    fn message_id_uses_from_domain() {
+        let msg = build(&from_mailbox(), &fixture("user@nel.pet", Some("s"), "b")).unwrap();
+        let raw = String::from_utf8(msg.formatted()).unwrap();
+        let line = raw
+            .lines()
+            .find(|l| l.starts_with("Message-ID:") || l.starts_with("Message-Id:"))
+            .expect("message-id header present");
+        assert!(
+            line.contains("@nel.pet>"),
+            "message-id should use From domain: {line}"
+        );
+    }
+
+    #[test]
+    fn missing_subject_uses_default() {
+        let msg = build(&from_mailbox(), &fixture("user@nel.pet", None, "Body")).unwrap();
+        let raw = String::from_utf8(msg.formatted()).unwrap();
+        assert!(raw.contains("Subject: Notification"));
+    }
+
+    #[test]
+    fn recipient_domain_extracted() {
+        let msg = build(&from_mailbox(), &fixture("user@Nel.PET", Some("s"), "b")).unwrap();
+        let d = recipient_domain(&msg).unwrap();
+        assert_eq!(d.as_str(), "nel.pet");
+    }
+}
diff --git a/crates/tranquil-comms/src/email/mx.rs b/crates/tranquil-comms/src/email/mx.rs
new file mode 100644
index 0000000..b04f192
--- /dev/null
+++ b/crates/tranquil-comms/src/email/mx.rs
@@ -0,0 +1,250 @@
+use hickory_resolver::TokioAsyncResolver;
+use hickory_resolver::error::{ResolveError, ResolveErrorKind};
+use hickory_resolver::proto::op::ResponseCode;
+use rand::seq::SliceRandom;
+
+use super::types::{EmailDomain, MxHost, MxPriority, MxRecord};
+use crate::sender::SendError;
+
+pub async fn resolve(
+    resolver: &TokioAsyncResolver,
+    domain: &EmailDomain,
+) -> Result<Vec<MxRecord>, SendError> {
+    match resolver.mx_lookup(domain.as_str()).await {
+        Ok(lookup) => interpret_lookup(
+            lookup
+                .iter()
+                .map(|mx| (mx.preference(), mx.exchange().clone())),
+            domain,
+        ),
+        Err(e) => classify_lookup_error(e, domain),
+    }
+}
+
+fn interpret_lookup(
+    items: impl IntoIterator<Item = (u16, hickory_resolver::Name)>,
+    domain: &EmailDomain,
+) -> Result<Vec<MxRecord>, SendError> {
+    let entries: Vec<_> = items.into_iter().collect();
+    match entries.iter().any(|(_, name)| name.is_root()) {
+        true => Err(SendError::DnsPermanent(format!(
+            "null MX record at {}: domain refuses mail",
+            domain.as_str()
+        ))),
+        false => {
+            let records: Vec<MxRecord> = entries
+                .into_iter()
+                .filter_map(|(prio, name)| {
+                    MxHost::parse(&name.to_utf8()).ok().map(|host| MxRecord {
+                        priority: MxPriority::new(prio),
+                        host,
+                    })
+                })
+                .collect();
+            match records.is_empty() {
+                true => implicit_mx(domain),
+                false => Ok(prioritize(records)),
+            }
+        }
+    }
+}
+
+fn prioritize(mut records: Vec<MxRecord>) -> Vec<MxRecord> {
+    records.shuffle(&mut rand::thread_rng());
+    records.sort_by_key(|r| r.priority);
+    records
+}
+
+fn classify_lookup_error(
+    e: ResolveError,
+    domain: &EmailDomain,
+) -> Result<Vec<MxRecord>, SendError> {
+    match e.kind() {
+        ResolveErrorKind::NoRecordsFound { response_code, .. } => match *response_code {
+            ResponseCode::NoError => implicit_mx(domain),
+            ResponseCode::NXDomain => Err(SendError::DnsPermanent(format!(
+                "domain {} does not exist",
+                domain.as_str()
+            ))),
+            other => Err(SendError::DnsTransient(format!(
+                "MX lookup for {} failed with {other}",
+                domain.as_str()
+            ))),
+        },
+        _ => Err(SendError::DnsTransient(e.to_string())),
+    }
+}
+
+fn implicit_mx(domain: &EmailDomain) -> Result<Vec<MxRecord>, SendError> {
+    MxHost::parse(domain.as_str())
+        .map(|host| {
+            vec![MxRecord {
+                priority: MxPriority::new(0),
+                host,
+            }]
+        })
+        .map_err(|e| SendError::DnsPermanent(format!("invalid recipient domain: {e}")))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn record(prio: u16, host: &str) -> MxRecord {
+        MxRecord {
+            priority: MxPriority::new(prio),
+            host: MxHost::parse(host).unwrap(),
+        }
+    }
+
+    #[test]
+    fn prioritize_sorts_by_priority_ascending() {
+        let result = prioritize(vec![
+            record(20, "mx2.nel.pet"),
+            record(10, "mx1.nel.pet"),
+            record(10, "mx1b.nel.pet"),
+        ]);
+        assert_eq!(result[0].priority.as_u16(), 10);
+        assert_eq!(result[1].priority.as_u16(), 10);
+        assert_eq!(result[2].priority.as_u16(), 20);
+    }
+
+    #[test]
+    fn prioritize_randomizes_equal_priority_order() {
+        let attempts: Vec<Vec<String>> = (0..200)
+            .map(|_| {
+                prioritize(vec![
+                    record(10, "a.nel.pet"),
+                    record(10, "b.nel.pet"),
+                    record(10, "c.nel.pet"),
+                    record(10, "d.nel.pet"),
+                ])
+                .into_iter()
+                .map(|r| r.host.as_str().to_string())
+                .collect()
+            })
+            .collect();
+        let distinct: std::collections::HashSet<_> = attempts.iter().cloned().collect();
+        assert!(
+            distinct.len() > 1,
+            "equal-priority MX order should vary across calls; got only {}",
+            distinct.len()
+        );
+    }
+
+    #[test]
+    fn implicit_mx_uses_domain_as_host() {
+        let d = EmailDomain::parse("nel.pet").unwrap();
+        let result = implicit_mx(&d).unwrap();
+        assert_eq!(result.len(), 1);
+        assert_eq!(result[0].priority.as_u16(), 0);
+        assert_eq!(result[0].host.as_str(), "nel.pet");
+    }
+
+    #[test]
+    fn no_error_response_yields_implicit_mx() {
+        let d = EmailDomain::parse("nel.pet").unwrap();
+        let err = ResolveError::from(ResolveErrorKind::NoRecordsFound {
+            query: Box::new(hickory_resolver::proto::op::Query::default()),
+            soa: None,
+            negative_ttl: None,
+            response_code: ResponseCode::NoError,
+            trusted: false,
+        });
+        let result = classify_lookup_error(err, &d).unwrap();
+        assert_eq!(result.len(), 1);
+        assert_eq!(result[0].host.as_str(), "nel.pet");
+    }
+
+    #[test]
+    fn nxdomain_response_is_permanent() {
+        let d = EmailDomain::parse("does-not-exist.invalid").unwrap();
+        let err = ResolveError::from(ResolveErrorKind::NoRecordsFound {
+            query: Box::new(hickory_resolver::proto::op::Query::default()),
+            soa: None,
+            negative_ttl: None,
+            response_code: ResponseCode::NXDomain,
+            trusted: true,
+        });
+        match classify_lookup_error(err, &d) {
+            Err(SendError::DnsPermanent(_)) => {}
+            other => panic!("expected DnsPermanent, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn servfail_response_is_transient() {
+        let d = EmailDomain::parse("nel.pet").unwrap();
+        let err = ResolveError::from(ResolveErrorKind::NoRecordsFound {
+            query: Box::new(hickory_resolver::proto::op::Query::default()),
+            soa: None,
+            negative_ttl: None,
+            response_code: ResponseCode::ServFail,
+            trusted: false,
+        });
+        match classify_lookup_error(err, &d) {
+            Err(SendError::DnsTransient(_)) => {}
+            other => panic!("expected DnsTransient, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn timeout_is_transient() {
+        let d = EmailDomain::parse("nel.pet").unwrap();
+        let err = ResolveError::from(ResolveErrorKind::Timeout);
+        match classify_lookup_error(err, &d) {
+            Err(SendError::DnsTransient(_)) => {}
+            other => panic!("expected DnsTransient, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn message_variant_is_transient() {
+        let d = EmailDomain::parse("nel.pet").unwrap();
+        let err = ResolveError::from(ResolveErrorKind::Message("transient resolver glitch"));
+        match classify_lookup_error(err, &d) {
+            Err(SendError::DnsTransient(_)) => {}
+            other => panic!("expected DnsTransient default, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn null_mx_is_permanent() {
+        let d = EmailDomain::parse("nomail.nel.pet").unwrap();
+        let result = interpret_lookup(vec![(0, hickory_resolver::Name::root())], &d);
+        match result {
+            Err(SendError::DnsPermanent(msg)) => {
+                assert!(msg.contains("null MX"), "msg: {msg}")
+            }
+            other => panic!("expected DnsPermanent, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn null_mx_alongside_real_records_still_permanent() {
+        let d = EmailDomain::parse("mixed.nel.pet").unwrap();
+        let real = hickory_resolver::Name::from_ascii("mx1.nel.pet.").unwrap();
+        let result = interpret_lookup(vec![(10, real), (0, hickory_resolver::Name::root())], &d);
+        assert!(matches!(result, Err(SendError::DnsPermanent(_))));
+    }
+
+    #[test]
+    fn empty_lookup_uses_implicit_mx() {
+        let d = EmailDomain::parse("nel.pet").unwrap();
+        let result = interpret_lookup(Vec::<(u16, hickory_resolver::Name)>::new(), &d).unwrap();
+        assert_eq!(result.len(), 1);
+        assert_eq!(result[0].host.as_str(), "nel.pet");
+    }
+
+    #[test]
+    fn valid_records_pass_through_with_priority_sort() {
+        let d = EmailDomain::parse("nel.pet").unwrap();
+        let mx1 = hickory_resolver::Name::from_ascii("mx1.nel.pet.").unwrap();
+        let mx2 = hickory_resolver::Name::from_ascii("mx2.nel.pet.").unwrap();
+        let result = interpret_lookup(vec![(20, mx2), (10, mx1)], &d).unwrap();
+        assert_eq!(result.len(), 2);
+        assert_eq!(result[0].priority.as_u16(), 10);
+        assert_eq!(result[0].host.as_str(), "mx1.nel.pet");
+        assert_eq!(result[1].priority.as_u16(), 20);
+    }
+}