Comment 6 for bug 1899485

This is not MailListNameExists :)

Anyway, what was your TB version before the update?
It would be great to find out which line is producing that error.
It should be either:
LookupAddressBook(recipientsList)
or
recipient.mDirectory->GetReadOnly(&readOnly);
Those could produce NS_ERROR_NOT_IMPLEMENTED that is then returned to the caller.

Are you using some special Addressbook type or an addon for it?

Does it happen on all replied messages or only some specific one that may contain some unusual recipient, e.g. a group ("name:;" or similar) ?