I was recently asked to try and fix a Canon imageRUNNER ADVANCE C5550i which was able to send scans via email, but only to certain email addresses. Even more confusingly, the users were able send scans to some Gmail addresses, but not not all Gmail addresses.
The issue here is that the Canon C5550i web management page does not clean any inputted strings, so a trailing space would be not be removed. Accidentally, the users were putting spaces after their email addresses (because after each word when you type, you normally hit the space bar). The web interface on the Canon C5550i Multifunction should have cleaned up any invalid characters, or at least not saved the email address if it contained invalid characters.
The fix of course was to notice and remove the trailing spaces – and voila, a working printer.