Registration E-Mails show up in bulk mail 1

[PanelMaker]

We have been having this problem for sometime. When users register at our site, or update their profile they receive an automated email informing them of their changes for security reasons or in the initial registration to receive their password. Every now and then we have clients call us and say they never received an email with their password, so we looked into the problem and found that on sites like yahoo mail, hotmail, gmail, etc. our emails show up in the bulk mail folder. I can only assume that this happens in some company email systems as well. So I guess my question is, how can I make it so that email servers don't see our emails as junk / spam / bulk?

 

[Troodos]

Avoid certain keywords (check text and subjects), avoid use of HTML and avoid file attachment(s) - if possible...

Brgds,

TrooDOS