Scenario
When using cloud providers for sending mail, there are inherent limits to sending mail.
Gmail : https://support.google.com/a/answer/166852?hl=en
2000 msg per day
10K total recipients per day
3000 unique recipients per day
These rates can change without notice, so JEMHC does not ‘try’ to stay within them. If JEMHC sends mail volumes that hit this limit, the mail-server can block the mail sending account. JEMHC reacts to send-failures by backing of in ever increasing amounts. After an hour, JEMHC takes the handler offline which re-enabled manually.
Options
Future implementation of https://thepluginpeople.atlassian.net/browse/JEMHC-2415 could allow the load to be spread over a set of accounts.
Use of SMTP relay features of some mailservers:
Gmail : https://support.google.com/a/answer/2956491#sendinglimitsforrelay
The maximum number of messages a user can send in a 24-hour period is 10,000. However, this can vary, depending on the number of user licenses in your Google Workspace account.
A registered Google Workspace user can't relay messages to more than 10,000 unique recipients in a 24-hour period.
Use an external mail service provider that is geared to high volume mail delivery.