Xero.com provide a feature which requires the use of an ! in the email address

Question

Unfortunately, this causes the following NDR when sending an email to it:

Server error: '550 Mailcore(2503): Restricted characters in address'

An example of the email address format is such:

'bills.!******@xerofiles.com'

I read that an exclamation mark is a valid character in the local part of an email address, but Office 365 doesnt seem to like it.

Is there any way around this? I can Admin my tenancy if required, although I havent got a clue how to do it as I set it up years ago LOL

Thanks