The short answer
Business email is usually filtered for one of three reasons: authentication records are missing or wrong, the sending address or server has a poor reputation, or the content looks like bulk mail.
Check them in that order, because the first is the most common and the easiest to fix.
Check authentication first
- A sender policy record that lists everything that sends on your behalf
- Signing configured and matching the sending domain
- A policy record telling receivers what to do with failures
- No conflicting or duplicated records
- Records covering every service that sends as you
The last point is where most problems are. A new tool sending on your behalf without being listed fails authentication and takes your reputation with it.
Then check reputation
| Check | What it tells you |
|---|---|
| Sending address on a blocklist | Delivery blocked outright |
| Sending server address listed | Shared hosting is a common cause |
| Domain age and history | New domains are treated cautiously |
| Complaint rate | Recipients marking mail as spam |
| Bounce rate | Sending to bad addresses harms you |
Shared hosting reputation is a frequent and invisible cause. Your mail can be filtered because of what other customers on the same server sent.
Test to several providers
- Send a test to accounts on the major providers.
- Check whether it arrived, and in which folder.
- Check the received headers for authentication results.
- Repeat after any change to sending arrangements.
- Include your transactional mail, not just marketing.
Point five is the one that matters commercially. An enquiry acknowledgement landing in spam costs you more than a newsletter doing so.
Separate your mail streams
Sending marketing and transactional mail from the same address means a marketing complaint problem can stop your enquiry notifications arriving.
Separate addresses or subdomains keep the reputations apart, and that separation is worth setting up before there is a problem.