I tried to apply your solution, unfortunately without success. if I complete the entry in the config.xml that you have listed and then restart the services, a backup file will be created automatically and all entries will be lost. The utf-16 problem is also very annoying for us because the pässler prtg network management is not able to interpret the subject and thus a monitoring of these mails is not possible.
please for exact instructions, as with the latest version of o365 backup the entries in the config.xml or proxy.xml have to be done.
Thanks and Greetings,