Very often you may find that you want to prohibit certain users from receiving mail from the internet. In the past, the solution would have been to give the specific users that are prohibited from ...
Published Jun 28, 2004
Version 1.0
Blog Post
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How to block all inbound Internet mail to specific users or distribution groups in Exchange 2003
The default send settings ...send denoting relay settings on the SMTP virtual server are by default" only the list below", and "allow all users and computers that successfully authenticate to relay".
In essence, your POP/IMAP users are relay users when they send using SMTP and are expected to authenticate. When they do, mail will not be denied from them.
However, if your internal POP/IMAP users are not authenticating to send mail, they're mail will be denied i.e result in 5.7.1 NDR. Most importantly it poses a security issue because SMTP relay is then anonymous (open to any client).
If you would prefer to deviate from the default settings on the SMTP virtual server relay config, then you can choose specific user (security principals) that have the right to relay mail. This would be your internal POP/IMAP users in your case.
In essence, your POP/IMAP users are relay users when they send using SMTP and are expected to authenticate. When they do, mail will not be denied from them.
However, if your internal POP/IMAP users are not authenticating to send mail, they're mail will be denied i.e result in 5.7.1 NDR. Most importantly it poses a security issue because SMTP relay is then anonymous (open to any client).
If you would prefer to deviate from the default settings on the SMTP virtual server relay config, then you can choose specific user (security principals) that have the right to relay mail. This would be your internal POP/IMAP users in your case.