Mail sending failed - sender thread limit exceeded
If you observe the below error traces in the logs,
.SMTPSendFailedException: 432 4.3.2 STOREDRV.ClientSubmit; sender thread limit exceeded [Hostname=XXXX.YY.prod.outlook.com]
Then refer the links below to resolve the same,
https://blogs.technet.microsoft.com/exchange/2011/04/11/store-driver-fault-isolation-improvements-in-exchange-2010-sp1/
https://www.michev.info/Blog/Post/2075/controlling-smtp-authentication-for-office-365-mailboxes