Forwarded email not received when sending to multiple email adresses

Hi,

I have forwarding rules setup with my domain. If I send a mail to all the aliases mail gets properly delivered. As soon as I send mail to multiple addresses only the first one receives the mail. Same happens when using the CC to add the extra mail address, it will only be delivered to the first email address in the address bar.  

So first I thought I cant send to multiple persons, but! if I use the BCC it gets delivered.

These are all the situations:

Mail to single address: Works
Mail to multiple addresses: Only the first receives the mail.
Mail to single address and a single address in CC: CC doesn't receive mail
Mail to single address and a single in the BCC: gets received by everyone
Mail to single address and multiple in the CC: gets received by everyone

Does this give anyone maybe a clue what might be wrong?

Cheers!