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Suppose that Alice sends an email to Bob using SMTP. We assume that Alice UA send her message to her mail server via SMTP.

Is there a difference between the SMTP message that Alice UA sends to her mail server MTA and the SMTP message Alice's mail server sends to Bob's mail server?

I tried to find examples on the web on SMTP workloads of a UA to MTA, and I did not find any. Most examples of SMTP workloads are between MTA and MTA...

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The SMTP protocol and the message contents are essentially the same for MUA-to-MTA and MTA-to-MTA. An MTA may add missing or extra headers and will append a Received header at the beginning of the headers, making it possible to trace the delivery path. Also, MUA-to-MTA connections may be authenticated and use a separate submission port with separate settings.

An incoming message is usually accepted immediately and then queued at the MTA. Most MUAs will show an error if the delivery fails because the server couldn't be reached. The workload is related to the queue. If there's plenty of mail to deliver, it's not possible to deliver it all at the same time. There might also be temporary problems with reaching the destination MTA, or temporary problems why it can't accept the message, like rate limits.

If the MTA gets an error that is temporary in nature, it will try the delivery later with increasing intervals, and the mail stays in the queue for the maximum queue period defined. After that, it will be considered as a permanent error and the mail will bounce back. The MTA might also give additional notifications on the delivery status.

The queue is an internal structure that handles a basic requirement in the SMTP model: as defined in RFC 5321, 2.1, a single MTA has the full responsibility for delivering the message at a time, until the next MTA accepts this responsibility:

Message transfer can occur in a single connection between the original SMTP-sender and the final SMTP-recipient, or can occur in a series of hops through intermediary systems. In either case, once the server has issued a success response at the end of the mail data, a formal handoff of responsibility for the message occurs: the protocol requires that a server MUST accept responsibility for either delivering the message or properly reporting the failure to do so.

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