mailer

Mailer is a term used in computing to describe software that creates, routes, or delivers electronic mail. In common usage it can refer to mail transfer agents (MTAs) that forward messages between mail servers, or to mail user agents (MUAs) that end users interact with to compose and read messages. Some systems combine both roles in a single package, while large organizations often run dedicated MTAs alongside client applications.

MTAs implement the Simple Mail Transfer Protocol (SMTP) to transfer messages, manage queues, and perform routing toward destination servers. They may also perform local delivery to user mailboxes or forward messages to other MTAs. MUAs provide composing, reading, and organizing features through interfaces such as desktop clients or webmail. Retrieval of messages from servers is typically handled by IMAP or POP3 in concert with the MUA.

Popular MTAs include Postfix, Exim, Sendmail, and Microsoft Exchange. Common MUAs include Mozilla Thunderbird, Microsoft Outlook, and Apple Mail. In addition, bulk or marketing mailers automate large-scale mailings for newsletters and transactional notifications, often with dedicated features for list management and analytics.

Security and standards play a central role in mailer design. Most systems support TLS encryption for SMTP and require authentication to prevent abuse. They implement anti-spam and anti-abuse mechanisms such as SPF, DKIM, and DMARC. Reliability features include queueing, retries, bounce processing, and mechanisms for handling undeliverable messages. Proper configuration is important to avoid open relays and ensure timely delivery.