Is it possible to retrieve emails from the time that we lose internet connection? Is there a way to just store those emails first when the server losses internet connection and just resend it or reprocess it after internet goes back?
The best thing I can think of is have a local relay and setup rules to retry and it also will keep the history (queue).
Epicor -> Your Local Relay -> Yahoo SMTP and then when stuff happens you can more than likely configure your local relay to act accordingly, heck even have a secondary SMTP if Yahoo kicks the bucket, especially if its business critical like ACH Remitance Notices etc… you might want to have failovers etc…
Also if you have your own Relay then you can also debug it easier, for example I had someone who used their own IIS SMTP but GoDaddy rejected all their emails and Verizon because their SPF was wrong, but it would have been difficult to diagnose without having verbose logs.