We have a service that requires a registration and the registration process sends some confirmation emails.
These emails say in a three-line message that you have to click some link with a generated token to complete the registration. We send these emails in several languages, always HTML-formatted.
These emails are always delivered instantly to Gmail, Yahoo and Hotmail accounts. It took quite some tweaking though to make our local Exchange server pass them through; the "intelligent" spam filter was always rating these emails as around 75% suspicious. Another mail server located on the same shared hosting sends emails that always get through. At the same time, other confirmation emails sent to our local Exchange server, usually get delivered to our mailboxes. The problem seems to be specific to our confirmation emails only.
The question is how do we make sure that these confirmation emails are not trashed by the spam filters? Is there a safe technique to, e.g. use some specific sender id like 'noreply' or including some specific text into the message, maybe more text or something else to let the spam filter classify the confirmation message as such.