Protocols
While popular protocols for retrieving mail include POP3 and IMAP4, sending mail is usually done using the SMTP protocol.
Another important standard supported by most email clients is MIME, which is used to send binary file email attachments. Attachments are files that are not part of the email proper, but are sent with the email.
Most email clients use a User-Agent header field to identify the software used to send the message. According to RFC 2076, this is a common but non-standard header field.
RFC 6409, Message Submission for Mail, details the role of the Mail submission agent.
RFC 5068, Email Submission Operations: Access and Accountability Requirements, provides a survey of the concepts of MTA, MSA, MDA, and MUA. It mentions that "Access Providers MUST NOT block users from accessing the external Internet using the SUBMISSION port 587" and that "MUAs SHOULD use the SUBMISSION port for message submission."
Read more about this topic: Email Client