If I'm not wrong, the XML config file domainList.xml (and every xml files basically) is not used anymore since v100(v17) of SmarterMail.
They were replaced by json files. You should edit this file instead
C:\Program Files (x86)\SmarterTools\SmarterMail\Service\Settings\domains.json
Also SmarterMail json files are flattened to one line, so viewing/editing them is a bit a pain.
I suggest you use an editor that can reformat it to a more human readable/editable formatting.
Visual Studio Code for example can do this after you opened the file:
SmarterMail will re-flat it to one line next time it save changes to it.
Of course be sure to backup the file before editing anything in case you need to rollback.
Kind regards,
Sébastien
Sébastien Riccio
System & Network Admin
https://swisscenter.com