Direct SMTP Engine

Built from Scratch in Rust

EuroMail's SMTP engine is a purpose-built mail transfer agent written in Rust using the Tokio async runtime. It is not a wrapper around Postfix, Exim, or any existing MTA. Every connection, every TLS handshake, and every SMTP transaction is handled by our own code, giving us full control over delivery behavior, retry logic, and performance characteristics.

Direct Delivery — No Third-Party Relay

When you send an email through EuroMail, our SMTP engine resolves the recipient's MX records and delivers directly to their mail server. There is no intermediary relay. Your emails do not pass through SendGrid, Amazon SES, Mailgun, or any other third-party infrastructure. This direct delivery model means fewer hops, lower latency, and complete control over the delivery path.

Automatic DKIM Signing

Every outbound email is automatically signed with DKIM (DomainKeys Identified Mail) using per-domain 2048-bit RSA keys. When you verify a domain in EuroMail, we generate a unique DKIM key pair and provide the public key as a DNS TXT record. The private key is stored securely and used to sign every message sent from that domain.

SPF Alignment

Emails sent through EuroMail originate from our dedicated IP addresses, which are included in the SPF record you configure for your domain. This ensures SPF alignment — the sending IP is authorized by your domain's DNS, and receiving mail servers can verify that EuroMail is permitted to send on your behalf.

DMARC Policy Compliance

With both DKIM signing and SPF alignment in place, emails sent through EuroMail pass DMARC (Domain-based Message Authentication, Reporting, and Conformance) validation. Whether your DMARC policy is set to none, quarantine, or reject, EuroMail-sent emails will satisfy the alignment requirements.

TLS-First Delivery

EuroMail prioritizes encrypted delivery. The SMTP engine attempts STARTTLS on every outbound connection. When a recipient server supports TLS, the connection is encrypted. Delivery status in the dashboard and webhooks indicates whether TLS was used for each message.

Connection Pooling and Retry Logic

The SMTP engine maintains connection pools to frequently contacted mail servers, reducing connection setup overhead for high-volume sending. When delivery fails due to temporary errors (4xx responses), the engine retries with exponential backoff. Permanent failures (5xx responses) are immediately classified as hard bounces and reported via webhooks and the dashboard.

MTA-STS (Strict Transport Security)

EuroMail supports MTA-STS (RFC 8461) for recipient domains that publish a Strict Transport Security policy. Before delivering to a domain, the SMTP engine:

1. DNS lookup — checks for a _mta-sts.<domain> TXT record containing v=STSv1; id=<id>.
2. Policy fetch — if the TXT record exists, fetches the policy from https://mta-sts.<domain>/.well-known/mta-sts.txt over HTTPS.
3. MX validation — verifies that each MX hostname matches at least one pattern in the policy (exact match or wildcard like *.example.com).
4. TLS enforcement — in enforce mode, TLS is mandatory and delivery to non-matching MX hosts is rejected. In testing mode, violations are logged but delivery proceeds. In none mode, the policy is effectively disabled.
5. Caching — policies are cached in memory for up to max_age seconds (capped at one week), reducing DNS and HTTPS lookups for repeat deliveries.

This ensures that when a recipient domain signals it requires encrypted delivery, EuroMail respects that requirement and never falls back to plaintext.