How does SMTP authentication work?

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tacit
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I asked a question this morning. Thank you for your reply. But my problem is as follows:

Suppose I own the domain 123.com and host it on inmotion shared hosting.

I set up 2 email address:
[email protected]
[email protected]

In order to send mail from [email protected] by SMTP, I need to login into Outgoing Mail Server: mail.123.com. This requires authentication. I can use [email protected] as user name and its mail password as password. The question is, after I login as [email protected], why can I also send an email from [email protected]?

Thank you!!
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My emails are bouncing back from [email protected] What could be the issue?
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Tim S.
Hi tacit,

Thanks for getting back to us. I'm more than happy to assist you today. SMTP authentication is there solely to validate that there is an account on the server thus providing access to send emails.

The from address is handled in the headers of the email and is not part of the SMTP protocol. SMTP authentication is designed to keep emails from openly being sent from a non-user account on the server. Once the account has been authenticated (when you log in), there's no other SMTP authentication that takes place.

You can send email from anyone after logging in. Who an email comes from is determined by the email headers, and SMTP Authentication does not scan email headers.

This is the same on virtually all email servers that use the SMTP protocol.

I hope this helps! If you need further assistance please feel free to contact us.

Thanks!

Tim S