I am getting that error message, what does it mean? Problem only with one recipient. Before mail was working successfully.
#550 5.2.3 RESOLVER.RST.RecipSizeLimit; message too large for this recipient
The maximum message size that's allowed 0mb. This message size 2mb.
It means exactly what it says, the SMTP server cannot deliver the message to that reipient because its configured not to. It is probably coming from the recipients SMTP server, not yours.
If the message is accurate then you should not be able to send them any emails at all but may be its not and its just that they can't accept ones that big, try sending a small message with no attachments and see if it gets through.
But I'm wondering this
"The maximum message size that's allowed 0mb. This message size 2mb"
Why allowed message size is 0mb?
I have try to find answer, didn't found, so that's why asking here
you would need to ask their mail provider that. it is nothing to do with Roundcube.