We have an email that is sent out to new members, confirming their username and password, but this is not being delivered to all email addresses. It seems to get stuck in some spam filters. Any idea on how to bypass this some way?
If you figure out an e-mail code that says "I am not spam and I am exempt from all filtering", there are lots of people hereabouts that would pay you big money for it ;)
The problem is that spam filters by their very nature happen at the recipient's end. That's why account information even from Big Name Companies includes a blurb about "If you don't get it within five minutes, check your junk-mail box".
Where is the mail getting stuck? There are three places.
#1 the easy one: the user's local spam filter #2 the ISP's optional pre-filtering. The user may or may not be able to turn this off, and they may or may not be able to use something like webmail to find any intercepted mailings. As when my current ISP decided to classify its own, individually requested mailings as spam. #3 the ISP's absolute filter. Remember cyberpromo? You can bet that no self-respecting ISP accepted mail from this sender-- not even to put it straight into the recipients' spam boxes. If you're getting filtered at this level, all you can do is talk to the ISP. They may or may not listen.
Spammers would love to know that same answer :) , so you're not going to find it all that easy.
To not get caught up as a false positive you can make sure to do a few things:
don't send too many messages (the more you send, the more likely it'll be flagged as spam) e.g.: rate limit sign-ups for the same account, from the same source IP, ...
make sure the headers of the email are complete (spammers often used to fail to do this properly, some anti-spam still checks on that).
make the content as little "spammy" in nature as you can. Write it neutral and factual, avoid all marketing flair. You've already sold them to get an account anyway.
Send it from a trusted source. E.g. if you host your website at big name shared hosting: odds are your IP range is already flagged as a source of spam due to your provider selling to offshore professional spammers. One way around this is to not direct deliver from your MTA, but to configure it to use a smarthost that's allowing you to relay messages - your service provider might offer that as a service - if they don't you can always build your own elsewhere (e.g. behind a business DSL line or so). Even if your provider offers you a smarthost: take care: it might be flagged as a source of spam regardless if they do not actively manage it appropriately.
Aside of that you do have to expect some messages to be flagged as spam. Fact of life.