As of February 25, 2020 Microsoft released Office 365 version 2002 and broke 3rd party applications from sending emails. They released a patch to version 2002 on March 1, 2020 to correct it.
Office 365 needs to be updated to correct the issue.
Version 2002, Build 12527.20242 released 3/1 is the correction.
Version 2002, Build 12527.20194 released 2/25 is defective.
https://docs.microsoft.com/en-us/officeupdates/monthly-channel-2020#version-2002-march-01
If you have questions, contact Microsoft or your IT professional on how to proceed.
