I downloaded and activated social sharing via email (share by mail was already active) but I still get the same error. Do you have another idea what could be causing the error?
We are also seeing this issue. Same versions and installation options.
Actually, some more looking around gave this thread which fixed my issues for now:
The problem I had found is after updating NextCloud, 16.0.* it looks like the config may have not converted correctly
in the log under settings was showing .
sharebymail Swift_RfcComplianceException: Address in mailbox given [Filebox@*******.com@localhost.localdomain] does not comply with RFC 2822, 3.6.2.
So to resolve this problem I changed:
Under basic setting I did find that the from email address had been filled in with the completed name@domain.com and the domain field had been filled in with the domain only.
After correcting this it was fine.
Not sure if this is or was your problem but Seeing I am not seeing a real answer to this problem. I am posting what I had to do to fix this same problem you had described.