This email address is a domain email address:
callcentre@domain.co.uk
I have no issues sending emails to any other email@domain.co.uk. I've narrowed the problem down to the resolving of the domain name.
I.E: If I type in callcentre@domain.co.uk outlook will get the user from exchange and load (what it thinks) is a resolved email address. I know this because as I click send, callcentre@domain.co.uk turns into Call Centre (The accounts display name).
It's not, however, resolving to the right address. It is instead trying to send to something like:
IMCEAEX-_O=FIRST+20ORGANIZATION_OU=First+20administrative+20group_cn=Recipients_cn=CallCentre@domain.co.uk
(I had replaced actual domain with domain)