Discovered a strange behavior (some might call it a bug ;) of MT’s notification feature. MT doesn’t seem to ensure that 8-bit mail headers are properly encoded (“7-bit clean”), as the following extract of an amavisd-new error message shows:
INVALID CHARACTERS IN HEADER
Non-encoded 8-bit data (char FC hex) in message header ‘Subject’
Subject: …o ‘Registration opened for OSCOM 4 in Z\374rich’\n
This nondelivery report was generated by the amavisd-new program
at host melon. Our internal reference code for your message
WHAT IS AN INVALID CHARACTER IN MAIL HEADER?
The RFC 2822 standard specifies rules for forming internet messages.
It does not allow the use of characters with codes above 127 to be used
directly (non-encoded) in mail header (it also prohibits NUL and bare CR).
If characters (e.g. with diacritics) from ISO Latin or other alphabets
need to be included in the header, these characters need to be properly
encoded according to RFC 2047. This encoding is often done transparently
by mail reader (MUA), but if automatic encoding is not available (e.g.
by some older MUA) it is the user’s responsibility to avoid the use
of such characters in mail header, or to encode them manually. Typically
the offending header fields in this category are ‘Subject’, ‘Organization’,
and comment fields in e-mail addresses of the ‘From’, ‘To’ and ‘Cc’.
Sometimes such invalid header fields are inserted automatically
by some MUA, MTA, content checker, or other mail handling service.
If this is the case, that service needs to be fixed or properly configured.
Typically the offending header fields in this category are ‘Date’,
‘Received’, ‘X-Mailer’, ‘X-Priority’, ‘X-Scanned’, etc.
If you don’t know how to fix or avoid the problem, please report it
to _your_ postmaster or system manager.
Such messages hence aren’t standard-compliant and might get rejected by some picky mail hosts (which is the correct behavior). This particularly limits the use of MT in non-US countries (where 8-bit characters are much more common)
Other things I found out (mostly trivial things, but eventually nice to know as they’re not always intuitive):
* Gentoo doesn’t seem to ship with the Mail::Sendmail Perl module. If you use Gentoo, don’t set “MailTransfer smtp” in mt.cfg as it probably won’t work. Instead either set “MailTransfer sendmail” or simply comment it out (as “sendmail” is the default). You don’t need to change or enable “SendMailPath /usr/sbin/sendmail” as this is the default setting which is fine for Gentoo.
* You only need to add your e-mail address to MT’s “Notification List” if you’d like to receive a notification when new entries are posted. Usually, this feature only makes sense for readers who don’t use an aggregator already.
* If you just like to receive notifications when new comments are posted, you only need to
1) tell MT to send e-mail messages through sendmail (see above)
2) Enable “Email New Comments” in your weblog’s “Preferences” screen (at the bottom of the page under “Comment Configuration”)
3) enter your e-mail address in your user profile (click on your username to get there)
4) Eventually make sure you only use 7-bit characters for the subjects of your entries and the name of your blog (as long as this issue isn’t fixed)
Finally, e-mail notification works for me :) IOW: I’m ready to receive all the comment spam twice ;) Probably time to install MT-Blacklist soon. 
 Update: I’ve just received my first comment spam as an e-mail message too, but guess what, spamassassin has correctly qualified it as being spam :) Soon, MT-Blacklist will be added to this blog.