Just to follow up, this topic is a duplicate. The request for multiple SMTP services has been requested previously:
The use case here is a little different, but the problem arose from a digest misconfiguration. Unix.com has 138062 users, a limit of 2000 emails per day even for mission critical stuff like password resets would only account for 1.8% of those users being able to interact on a daily basis.
edit: corrected from 2500 to 2000, to reflect the actual limit