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Add support for a user-configurable whitelist, preferably by email user domain. The whitelist in my view would continue to check email addresses for correct user/host pairs, but would NOT perform the more advanced username and HELO/MAIL FROM/RCPT TO checks.
The reason (at least in my case) is this - when using shared hosting or cloud servers, your server is going to be in a range with other cloud servers. Sometimes those servers are added to a blacklist by mail service providers. When you connect, they reject the mail connection outright. However, the email is a valid email address. And in my case, I use an upstream mail provider to send emails, so any email that is actually destined for the user would arrive properly.
What this means is:
Drupal machine -> Remote mail server = rejected (email_verify fails the check - which in my case means their requested invite is rejected.)
Drupal machine -> Mail service provider -> Remote mail server = accepted, so email to them actually works properly
The net result is a failed check when it really shouldn't have failed.