AWS SES - Custom MAIL FROM and From addresses.

Think of an email using a postal letter analogy. You write your letter and put it in an envelope and the post service delivers it. In SMTP, programs called Mail Transfer Agents (MTA) are the post service, and they transfer the messages. The messages have an envelope part and the "letter" itself, which consists of headers and the message body, just like a written letter has a date and inside address (the header) and the salutation, body, and closing (the body).

During the SMTP protocol negotiation between MTAs, the sending MTA communicates details of both the envelope and the letter. The "return address" is send using the MAIL FROM verb form, and the outside recipient address(es) are sent using the RCPT TO verb form. These addresses may or may not correspond to the addresses in the "letter" portion, which is transmitted using the DATA verb form. Sometimes copies of the "letter" are sent to multiple MTAs to be delivered to users local to those MTAs, so the RCPT TO addresses will only be for the local deliveries, but the "inside addresses" (the To: and CC:) headers, will list the entire distribution of the letter. BCCs are handled by addressing only via RCPT TO and not on a header To: or CC: line.

In SES, AWS uses the MAIL FROM address to handle "returned mail," also known as a bounce or non-delivery report. By segregating the Return-Path to a dedicated domain, SES can implement a custom incoming mail receiver to handle returned message notifications and then forward them to the original message submitter either via SMTP relay or via the feedback notification capability to a SNS topic.