Hi Til,
Thanks for your quick responses
Unfortunately the MX record solution isn't an option for us because we use a 3rd party antispam solution which only accepts email for on domain1.tld and will reject anything for domain2.tld....hence why we need the email re-addressed to
user@domain1.tld
I think I'll look at doing this outside of ISPConfig. Once I find the solution, I will post here so this thread helps other users who might be in the same position as me.