1. login via Putty (ssh client) user : ron
2. pine
3. compose
4. mail to an address i.e. ron@girder.nl
5. type subject and body
6. control-x (send)
7. Now I'm getting the following error:

[Mail not sent. Sending error: 553 5.3.0 <ron@ns3.plushosting.nl>... No]

Anyone any idea?

Error 553 is a rejection error by the mail server. This can happen for a number of reasons. You need to get the full error message to really find out what is happening. This could be a relay issue, a mis-configured domain issue, etc.

Try using a mail-agent like outlook, pegaus, or eudora or look in the logs to get the full error and post it here.