Support System isn't sending emails (still)

Hi, support system doesn't send email notifications. This issue has been reported many times, and this topic from 5 years ago with a possible solution has gone unresolved for 5 years.
https://premium.wpmudev.org/forums/topic/i-have-found-why-the-support-system-was-not-sending-email

I have enabled SMTP and all mail functions other than support system works perfectly. The topic above seems to suggest that the issue is related to the username used for sending for some reason.

Any help would be appreciated.