Q2. That is independent of Moodle. It is the job of the MTA admin to keep it efficient and clean. The usual SPF, DKIM, black lists, etc. story.
Q3. It is possible that a) Moodle peaked at generating those e-mails, or b) the MTA was unable to receive (and spool) at the rate Moodle was handing over c) the MTA could not relay the wave of mails. Its log files and bounced mails should contain the answers.
Documentation: Couldn't find much other than https://docs.moodle.org/39/en/Messaging_settings#Email. There are tons of discussions about sending forum mails in various forums on this site. Try the Advanced search.
P.S. This is not strictly a performance issue, it is about SMTP servers. Better suited under General help.