How to prevent email with username+password being treated as junk

This is an important topic. If a user's first email from the site gets marked as spam, it's a bad first impression. MailChimp (my favorite email marketing service) recently updated their articles on how to avoid spam filters generally and in Apple Mail specifically. Basically, spam filters assign points to potential spam elements they find in email messages. If the message gets too many points, it is marked as spam.

So, I ran the two default Drupal emails through Litmus's spam check (used by MailChimp and others). It tests "all major Spam filters." I chose two random Drupal account creation emails (1 of each type). There were two fails:

Fail (no suggestion): html validation (they are plain text email, so this is to be expected, right?)
Fail/Suggestion: "Your email has no DKIM header. Postini holds DKIM support in high regard, you should consider adding it." More information on DomainKeys Identified Mail (DKIM) can be found here: http://www.dkim.org/index.html

Two other notes:

1. Links: All links may get filtered automatically by certain spam filters, as @ElegantSolutions mentions above, but it will be less likely if the From address and reply-to email addresses are the same andthey match the base URL of the link.
2. Mark-up: The default is plain-text, so any mark-up involves overriding the default system email. That's OK I guess, but in my experience people simply don't read their email. So if you choose a custom html email, keep it short, sweet, and run it through a Validator.

Here are some specific Drupal things you can do to reduce the number of spam points the email receives:

1. Make sure your site's email address (the one at /admin/settings/site-information) at matches the URL of the site.
2. When admins create account for users, make their user name something that looks like a real name (an account for user named David won't earn any points, but CrazyDood76 might). Using names is only mentioned in the Apple Mail article, but I think it makes sense for other email providers as well.
3. Because spam filters vary greatly, the only guarantee is to have users white-list the site's email address or the phrases "An administrator created an account for you at" and "Account details for." You can create a block on the /user page and customize the text in that first email to instruct users to do so. People will probably not read it, but at least you'll have it somewhere. :-)

You're welcome. 15% seems to be a particularly high fail rate, which makes me think that something else is amiss (emails and link not matching, etc.) so simply removing that hyperlink may not end up helping (you can try changing it to /user and then including the user/pass combo as text).

I suggest running the emails through Litmus first, then you can target exactly what's going on. It's not cheap, but you can create an account and then just cancel it in 7 days.

Thanks again!
Whitelisting may well be the most reliable defence. It is unfortunate that it puts the onus on the end user.