Mail Attachment Problems

I Have problems sending files with mail.

When sending attachments even as small as 300kb (And I can forget about sending files over 1mb) .I usally get an error message saying: cannot send message using server named (then the name).
And it is the correct server name. All the other account info is correct to.
It does not come up every time, when sending small files but always when sending several or bigger files. When sending tekst mails only, there`s never any problems.


Anyone know how to fix this?