How to reduce probability that valid/wanted/solicited emails are trashed into SPAM folder?

there are some tips:

- Use online IP SPAM blacklist checker to check if IP from which email is sent is not blacklisted
- Use online SPF & rDNS checker to check if proper SPF record is set and server hostname points back to IP (reverse DNS/PTR record)
- Use SMTP instead of PHP mail(), make sure SPF and Domainkeys are set up correctly
- Some spam filters see emails with a lot of links as spam (it's just one criteria)
- Ask users to add your email address to their mailbox Contacts list.
- Ask users to click "Not Spam" button to alert Gmail that messages sent from you are solicited.
- Sending promotional and transactional email from different email address or domain
- change new account emails Subject to something like "YourCompany New client account"
- Explicitly indicate the email address subscribed to your list in mail body.
- Tell users on how to Unsubscribe from future email