[support] Problem with Contact us form

[Afan Pasalic]

Hi,
I have a weird situation and have no idea what to do.

On my Drupal 6 Contact form page (the simple one, without any additions) 
, as Categories I put names and in Recipients field their email address. 
As it should be.
A visitor will select the name, enter question and after the visitor 
submitted the Contact form, email should arrive to selected person's Inbox.
Though, if I select myself (and email is the one above, the one I use in 
this email) I'll get the contact. But if I use FLast at UKexample.co.uk the 
email/contact will never reach the destination. The email 
FLast at UKexample.co.uk is valid because if I send an email to this email 
by Gmail or any other way - it will get there. Looks like Drupal doesn't 
accept UK email format (.co.uk) :-)
Any idea how and/or where I can check what's causing the issue?

Thanks,
aFan