Here is what your ISP says you should check:
Under Outgoing mail (SMTP), make sure the following is entered: smtp.onvol.net or mail.onvol.net
All other options in the current dialog should be cleared (i.e. inactive)
Click Advanced
Under Server Port Numbers, SMTP port should be 25, POP3 port should be 110.
Make sure all other options are cleared (i.e. inactive)
There is a lot of information about the error codes etc at
ONVOL's website.
This is assuming you havn't already been there.