When registering a new member, i got an error message that said mail could not be sent because validateSingleEmailAddress failed.
I also found that i could not delete my MailHost object's ESMTP loginname and password, neither through the Plone interface nor through the ZMI.
This link provided the answer: Throw away your MailHost, and replace it by a SecureMailHost with id 'MailHost'. Which works!
Although i still cannot empty the ESMTP settings, but that doesn't keep me from registering members.