Some emails not coming through ?

[Anthony2oo5]

Hi guys, I hope you can help.

Iv recently purchased a VPS and installed cPanel for our business to host its websites and our main priority is emails.

I seem to have got everything working and I have an external support company looking after it for me but their response times arnt great.

Let it be know that im also not an expert at cPanel / WHM and linux, Im more Exchange, but iv adapted my knowledge as much as I can and got this far.

The problem im having is some email seem to come through and some dont. Im completly stumped now.

Domain1 (gmail account)
Domain2 (cpanel on my VPS)
Domain 3 (cpanel on another hosting provider)

Domain 1 can send to both domain 2 and 3, and vice versa
Domain 2 can send to 1 and 3
Domain 3 can send to 1 but NOT 2
Domain 2 can send to Domain 2 via webmail, but if I try via Outlook IMAP it dosnt come through, and I dont get an NDR.

If I send an email to an email address that dosnt exsist, I DO get an NDR.

can anyone help with this please ?

Thanks in advance for your time.

- - - Updated - - -

I just thought on, where I said " Domain 3 can send to 1 but NOT 2 " Iv only tried that with Outlook via IMAP, I havnt tried that with webmail.