I sent an email with a 6 to 8mb attachment and got this message. Did the same minutes later using my gmail account and no problem. How do I set size to allow for large attachments, please?
It is not completely clear to me if this error is coming from your Plesk mail server or from a third party mail server that you were sending the message to?
On Plesk's side, you can change the maximum message size under Tools & Settings -> Mail Server Settings -> Settings.