short email domain problem

Hi, I have setup a email on a new server using cpanel. Seems like I can't send emails to email address that the domain is short like aol dot com or even some with an hyphen like on-line dot ca. I have no problem receiving from those emails but when I send it doesn't work and when I look at the email log it gives a 505 error. Like today I couldn't email an email address that had maine.rr.com as the domain. The email was never received. Would anyone know anything about this?

I have a site on my server that when people register, they receive a confirmation email to validate their account but seems like most of them with an email with a short domain doesn't seem to get the confirmation email.

Again, I send emails from the short one to the one on the server no problem but not the other way around and that from the site and also outlook.

Those who dont receive the email can reply back, I didn't receive it but even if I reply, they don't receive mine.

Appreciate your help and value your time.

Regards,

David

 

So far today all the ones that registered using aol.com and rr.com didn't get the activation email, everyone else did. Any idea?

Thanks in advance,

D.