Mail Transfer Agent (MTA)
Server software responsible for routing and delivering email messages between mail servers.
A Mail Transfer Agent (MTA) is the software that runs on mail servers and handles the actual routing, queuing, and delivery of email messages using the SMTP protocol. Popular MTAs include Postfix, Sendmail, Exim, Microsoft Exchange, and cloud-based services like Amazon SES and SendGrid.
How MTAs work in the email delivery chain: 1. The sender composes an email in their Mail User Agent (MUA) — e.g., Gmail, Outlook 2. The MUA passes the email to the sender's MTA 3. The sender's MTA looks up the recipient domain's MX records in DNS 4. The MTA connects to the recipient's MTA via SMTP and delivers the message 5. The recipient's MTA passes it to a Mail Delivery Agent (MDA) for inbox storage
Why MTAs matter for verification: During email verification, SendSure's engine communicates directly with the recipient's MTA through an SMTP handshake. The MTA's response codes (250 for valid, 550 for invalid, 4xx for temporary issues) are critical signals in determining whether an email address is deliverable. Different MTAs behave differently — some are transparent, some use greylisting, and some are configured as catch-all — which is why a multi-stage verification approach is essential.
Related Terms
SMTP Handshake
The process of connecting to a mail server to verify if a specific mailbox exists, without actually sending an email.
MX Record
A DNS record that specifies the mail server responsible for accepting email for a domain.
Greylisting
A spam prevention technique where a mail server temporarily rejects emails from unknown senders, expecting legitimate servers to retry.
Want to learn more?
Read our in-depth blog posts on email verification and deliverability.