Keep in mind that it is possible for legitimate messages to trigger spam filtering, and it is possible for legitimate companies to be publicly blacklisted. There is a chance that DNS or server settings on the recipients end may be causing delivery issues. In short, a number of factors determine whether a message is delivered to your Inbox, your Junk E-mail folder or if it is blocked by your server as spam.
Here are a few guidelines to remember:
- If an email has enough characteristics of spam, it will be moved into the Junk E-mail folder.
- If an email has strong characteristics of spam, it will be deleted at the server level before ever reaching your inbox.
- If the sender's DNS records are not set up properly, they may be inadvertently violating their own security protocols, resulting in delivery rejection/failure.
- Misconfigured mail servers are also blocked by your server. Most misconfigured email servers do not have reverse DNS or they are trying to spoof (forge) a domain name.
Check your incoming mail server
Depending on the results from the previous step