Sender denied
Question / Problem
While trying to send an email a "Sender denied" error occured.
Answer / Solution
Your Exchange Server returns this error if the sender address is blocked by the sender filter of the Exchange Server: https://docs.microsoft.com/en-us/Exchange/antispam-and-antimalware/antispam-protection/sender-filtering?redirectedfrom=MSDN&view=exchserver-2019
Please deactive this filter or at least exclude SmartPOP2Exchange from ever being rejected.