I am having this issue that when I send an email using rc it does not get saved in the sent folder, and I get this error as well. After testing, I noticed the following:
1. If attachment size is less than 700 kb (can't be 100% exact), it's being saved.
2. I can save drafts with much bigger attachment, no problem.
3. The email is actually sent, but not saved in sent folder.
4. I tried using Outlook, got no issues in sending/saving.
5. In sent folder, for messages sent using outlook that have attachment with big sizes (say 4 mb), I can see them there.
I even followed an issue which has been posted by someone else here facing the issue, but with no help, since the latest version already have the fix.
Mail server used: hMailServer (latest).