We called this as false-positive. False positives are innocent emails that get mistakenly identified as spams. Recent mail system security has incredibly tighten due to number of spam pattern increase. Before your email being delivered to respective mailbox, the email being filtered based on recipient mail server rules.
Example of filtering that can happened in recipient mail server:
1. PTR checking (pointer record or Reverse DNS)
2. SPF checking (Sender Policy Framework)
3. Bayesian Filtering
4. SpamAssassin Server Scoring and Filtering
5. RBL (Real-time Blackhole List)
What we can do?
Depending on how tight is the filtering level, false-positive can happen in any mail server. This is quite annoying since you cannot do anything from your side to fix this. What you can do from your side then? You can use following tips to bring up email’s reliability:
- PTR – You must use a SMTP server to relay your email to the recipient. That server must have a public IP which recipient can see. That public IP must have a reverse lookup value. Example:
- Public IP: 126.96.36.199. Hostname: mail.myserver.net
- When you lookup mail.myserver.net, you surely can get 188.8.131.52, but when you reverse lookup 184.108.40.206, do you get the same result (mail.myserver.net)?
- How to create PTR records? You MUST contact the IP owner, which can be found from whois page.
- SPF – This one is useful to tell the world that your domain’s email address should come from certain IP address. Every spammer can use your domain as “FROM:” field, SPF checking will make sure the domain send from, match the sender IP specified in SPF record. You can generate the SPF records from OpenSPF and apply into DNS records of your domain (TXT records).
- DKIM – This is quite new technology, where sender prove the email comes from them by signing the email with digital signature. You can browse around to see how to enable DKIM for your domain/server.
- dnsbl.info – Make sure your SMTP IP address is not listed in this website, http://www.dnsbl.info/ . This website can tell whether your IP is in any RBL list or not. If listed, contact the anti-spam organization that list your IP and request for removal. You might need to follow their requirement for that.
- Click “Not Spam” – Usually, if the sender is using a new domain and do not have any transaction with that particular mail server previously, it will mark your email as spam especially for email service provider like Yahoo, Gmail and Hotmail. If it happens, make sure you click “Not Spam” or “Not Junk” to let the the mail server know that this is a valid email and should be sent to inbox.
If you have done everything as list above but still cannot pass through the inbox, something is not right on the recipient side. Contact their system administrator and let them know about this so they can whitelist you inside their server.
Leave a comment if you have more point to share. Cheers!