Accounts

I've sorted out the account signups for this here blog, so you should be able to sign in / retrieve passwords etc. now. See below if you're really interested in what the problem was. The blog software has also been upgraded to the latest stable release, but it shouldn't make any difference from where you're sitting. Anyway, normal people can stop reading.... now.

(I'm posting this, because I've seen a number of people on forums who've had the same issue I did, and none of the threads were answered. Hopefully Google will pick this post up.)
The main problem with the signups was that the return address hadn't been specified in MT, so the emails weren't being sent out to external addresses. Unfortunately, MT doesn't appear to email errors to the admin, so I didn't know about this until Koof pointed out that he couldn't sign up. Once that was fixed I created a new account with my work address as the email address, but the email bounced off the Exchange server with a 500 error for an invalid Return-Path.

MT (on Linux at least) sends mail through the sendmail utility (Postfix in my case) as the webserver user, so it wasn't setting the envelope address sensibly. This meant that everything appeared to be coming from the (unresolvable) local-network hostname, which was correctly rejected as invalid by the remote side. To fix this you can set up Postfix to re-write the envelope headers by editing /etc/postfix/generic, and adding, e.g.:
webserveruser@localhostname.localdomain webmaster@example.com
Then add:
smtp_generic_maps = hash:/etc/postfix/generic
to /etc/postfix/main.cf if it's not already there, then run:
postmap /etc/postfix/generic
and
killall -HUP master
to make the changes stick. Once that's done, mail from the webserver user will appear to come from webmaster@example.com and all should be well.