Mailrouting

Mailrouting is the process by which electronic mail is delivered from one sender to another across computer networks. It relies on a chain of software components called mail transfer agents (MTAs) and mail delivery agents (MDAs), in combination with domain name system records, to determine the path a message should take.

In typical Internet mail, the sender's MTA hands a message to its outbound MTA, which then performs a DNS MX lookup for the recipient's domain to find the destination MTA. The MX records indicate one or more hosts and priority values. The sending MTA connects to the selected destination MTA via SMTP and transfers the message. If no MX records exist, some systems fall back to the domain's A record. If delivery cannot be completed immediately, the message is queued and retried according to policies.

Routing may pass through relay or gateway servers, especially for organizations with centralized mail infrastructure. Edge security and content filtering, TLS encryption via STARTTLS, and authentication mechanisms such as SPF, DKIM, and DMARC influence whether messages are accepted or flagged as spoofed or spam. Internal routing may also apply to mail within an organization, using mail routing rules, aliases, and distribution lists.

Reliability considerations include queue management, backoff timers, and bounce or nondelivery reports. Common issues include DNS misconfigurations, misconfigured MX records, relay loops, blacklists, and performance bottlenecks in MTAs. Effective mailrouting aims to balance delivery speed, security, and compatibility across diverse mail domains.