Redmine

Microsoft office 365 smtp server has made some changes that will cause
this error message

Email delivery error: 432 4.3.2 STOREDRV.ClientSubmit; sender thread limit exceeded
basically

New throttling limit for concurrent connections that submit messages
The service has various limits to prevent abuse and to ensure fair use. An additional limit is being added. Under the new limit, up to three concurrent connections are allowed to send email messages at the same time. If an application tries to send more than three messages at the same time by using multiple connections, each connection will receive the following error message:

432 4.3.2 STOREDRV.ClientSubmit; sender thread limit exceeded

https://docs.microsoft.com/en-us/exchange/troubleshoot/send-emails/smtp-submission-improvements

other projects have worked around it by introducing a
maxConnections: 3 limitation to there mailers
ex

https://support.sisense.com/hc/en-us/community/posts/360029660673--Custom-SMTP-Email-Server-Solution-for-Outlook365-error-432-4-3-2-STOREDRV-ClientSubmit-sender-thread-limit-exceeded-

my setup

Environment:
Redmine version 4.0.2.stable
Ruby version 2.6.1-p33 (2019-01-30) [x86_64-linux]
Rails version 5.2.2
Environment production
Database adapter Mysql2
Mailer queue ActiveJob::QueueAdapters::AsyncAdapter
Mailer delivery smtp