Unroutable mail domain

This can occur for several reasons.

Make sure you have the proper spelling of the email address and no extra spaces.

The recipient mail server may be having problems.

If you have sent more than 150 emails in an hour this will occur. Accounts are limited to 150 emails an hour to limit spam abuse. You will want to throttle the application to 150 an hour or perform 150 at a time.
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