New user email ends up in Spam folder

When someone registers on my site their registration email, which contains their password, ends up in the spam folder. This has started happening for gmail, hotmail, yahoo and it didn’t use to happen.

I’ve checked the server isn’t black listed. I’ve tried using SMTP. The from email address is the same as the website. I haven’t sent any bulk mails. I’ve changed the email content so it’s friendly, not too basic and has no negative spam words.

Anybody got any other ideas for things I can try?

Thanks