contact form does not send e-mail within the domain (no problems outside of the domain)

Hi everyone,

I already asked about this once before, but wasn't able to solve the issue. The old thread is here: http://drupal.org/node/159707
I guess it's probably best to reply there. I was reluctant to start a new thread, but don't know what the best practice is in such a case.
The problem is exactly what is stated in the title. Sending mail works fine except when the address is in our domain.

Our club's site is really not busy, but once every month or so we get an e-mail through the contact form and I really would prefer to send that to the real club addresses.

Thanks everyone for helping out with this.

If someone else uses gmail for their e-mail solution (with or without problems), I'd love to hear about it.

Robin