Mac Mail in OS X 10.10.4 or 10.10.5 has had significant problems for many users at many ISPs. We have particularly seen problems with authentication, and believe they are related to changes Apple made to let Gmail two-factor auth users connect without requiring app-specific passwords. If you are having problems sending mail via smtp.pobox.com, particularly if you also have Mac Mail set up to access your Gmail account, you may need to do the following:
- Delete mail client accounts from Internet Accounts
- Delete mail accounts from Mac Mail Preference
- Delete all of your SMTP server settings
When all of the configurations have been removed, set up your outgoing server. If you have a Mailstore account, set up both your incoming and outgoing server settings
For your SMTP server settings use:
- Server: smtp.pobox.com
- Port: 587
- Authentication: Password
Then add your other client server settings back. If you want to use our SMTP, select it from the drop down menu in the Preferences section for Mac Mail. DO NOT change the server on a Gmail account's SMTP settings to be smtp.pobox.com -- even though everything looks correct in the user interface, it mangles your credentials.