Alias email address is not working in SBS 2008

did you ad otherdomain.co.uk to the accepted domains list?
you need to add it before an external sender can use the additional domain
internally the smtp address will resolve to a local mailbox

Are you saying that this alias is getting an NDR from *one particular sender* or all of them ?

@endital's suggestion is the first thing you should check.
if @otherdomain.co.uk is already in accepted domain list, I'd rather go check the sender's end if s/he is pulling up the wrong email address / spelling mistakes.

Only one sender (julie@companyB.com) has problems and their domain has been added to safe sender on the receiving computer's Outlook 2003.
The mailbox is jimbrown@otherdomain.co.uk, but some emails use an old address of jim.brown@otherdomain.co.uk and so I set up an alias in SBS 2008 to redirect these emails.
If I send an email to jim.brown it goes through the jimbrown mailbox, if julie@companyB.com sends it to jim.brown she gets an NDR, user unknown.It is rejected by the SBS 2008 server.
If Julie sends to jimbrown it's ok, but companyB.com is not being refused.
sunny - yes just one sender, and I checking her address book, it does say jimbrown. I think when she may have a second entry with the dot, but why isn't it being redirected?
 

Sunny, I'll try that when I can get access to the PC, any idea why the SBS 2008 is rejecting the alias from Julie and accepting it from me - sorry to be a pain in the rear, but more concerned with this aspect. I really appreciate your help.

I dont think its an SBS issue. It's probably an outlook issue.

if SBS/Exchange rejects the email address, it will be for emails from all users.

did it work mike ?