How to prevent user/pass email being seen as junk

This is about that first email that is sent out by Drupal sites to newly registered Drupal users, welcoming them and telling them what their initial user name and password are.

We've found that:
1) these emails often end up in people's junk/SPAM folder (depending on the email client and SPAM filters used)
2) people don't check their junk/SPAM folder enough, so many never return to our site

Apart from more clearly stating on the registration form that newly registered users should check their junk folders, we've been wondering what can be done to reduce chances of this happening?

Things that may or may not be of influence:
o the subject of the email
o the wording of the email body text
o the hyperlinks in the text (i.e. the login link)
o the mark-up
o the sender/reply-to address
o ...

Enlighten us!