When should the Sender header and From header be set?

[kikaragyozov]

I have an SDK wrapper on top of MailKit that handles SMTP-related things, like authentication, sending of emails etc.

1. If authentication is required, and Username and Password are specified, and the Username is an e-mail address:

   • Use the Username as the value for the From header.

2. If authentication is required, Username is an e-mail address, but the client has explicitly specified a value for the From header:

   • Set the Sender header to the Username.
   • Set the From header to the client's specified value.

3. If authentication is required, but the Username is not an e-mail address, and the client has not specified a From header:

   • Throw an exception.

4. If the client has specified a value for the From header:

   • Set the From header to the specified value.

Are all of these states valid and sound in reasoning? Or is something flawed here?

[jstedfast]

1. Sounds reasonable.
2. I would recommend only setting the Sender if the Username value differs from the From value specified by the user.
3. Yep.
4. Yep.

To elaborate a bit, the Sender header should only ever be used if it differs from the From header.