You seems got this error at Roundcube or Horde.
Here is explanation how it works:
Supposedly you’re trying to send message with an attachment file 44M
And you have the message_size_limit set in 50M as explained here
And you will not be able to send such message because attachments in a message are encoded by base64
It means your message will be by 30% bigger than expected, please see below calculation
You cannot send such message due to the limit in 50M, so you need either rise message_size_limit or upload and share your attachment to services like Gdrive/Dropbox/etc
- None Found