We have always used Thunderbird for the email. I recently set up email for a friend with AT&T and AT&T also required Port 587. We can't send emails using Mapi because Thunderbird reports that no recipients have been designated (untrue) and the email fails. This also happened before the port change. The Direct method worked perfectly in the past and saves a lot of time since we just tell it to print to email the entire range of that day's invoices (after checked the reprint box) and it would work unattended.

The ISP is targeting spam with this change.