Check that your sending domain is authenticated, send a real test to see whether it lands in the inbox or spam, and fix whatever the test flags: DNS records, blocklists or spammy content. Warm up new mailboxes gradually, remove addresses that bounce, and monitor placement regularly rather than testing once.
Here's a deliverability problem that creeps up on growing businesses. You start with one email platform and an SPF record that passes. Then you add a CRM that sends email. Then a helpdesk. Then a newsletter service, and a tool for invoices. Each one asks you to add an include to your SPF record, and each addition looks harmless. One day your mail starts landing in spam, and every individual include still looks fine.
How a growing SPF record quietly breaks
SPF has a hard ceiling on how much work a receiving server will do to check it. RFC 7208, the SPF standard published by the IETF, requires evaluators to limit the include, a, mx, ptr and exists mechanisms and the redirect modifier to 10 DNS lookups in total. If a record goes over that limit, the result must be a permerror.
That's the trap. Adding every sending service to your SPF record can break SPF altogether. Each include you add can trigger lookups of its own, so a record with includes for a marketing tool, a CRM, a helpdesk and a newsletter service can run past the limit even though each include is valid on its own. Nothing about the record looks wrong when you read it. It fails because of the total.
A permerror means the record can't be interpreted correctly, which is the opposite of what you added it for. Mailbox providers use authentication records to decide whether to trust a message, so a broken SPF record undermines everything else you do to stay out of spam.
Count your lookups before you add another sender
The practical fix is a habit, not a tool: run an SPF check every time you add a new sending service, and before you add it if you can. Several of the tools further down have SPF checkers that will flag a record in trouble.
- Keep a short list of every service that sends email as your domain, and why.
- Remove includes for tools you've stopped using; old includes still count.
- Check the record after each change, not just when something breaks.
- Ask a new vendor whether they need to be in your SPF record at all before you paste their include.
If you'd like a plain-English primer on the record itself, this explainer on what an SPF record does and how it's read covers the basics.
Seven fixes in the order that finds problems fastest
- Check your authentication records. SPF, DKIM and DMARC for the domain you send from. Mail-Tester offers checkers for all three plus a DNS record lookup.
- Send a real test. Send an ordinary email to a placement or spam-test tool and read where it lands. GlockApps tests placement across Gmail, Outlook, Yahoo and others before you send to your list.
- Fix what the report flags. A failing record, a blocklisted IP or spam-trigger wording. Start with the quickest cause, which is usually a DNS record. Folderly checks sending IPs and DNS alongside placement.
- Warm up new mailboxes. Build up any new mailbox or domain gradually instead of sending your full volume on day one. MailReach and Warmup Inbox both offer warm-up. A sender with no history looks the same as a spammer to a filter.
- Clean the list. Remove addresses that bounce or never engage before the next campaign. This one's housekeeping inside your own sending platform. Mailing dead addresses damages the sender reputation of the whole domain.
- Keep your copy simple. A clear sender name, one main call to action, and a plain-text version that matches the HTML. Content is one of the factors spam tests score, and Mail-Tester's score shows you where you stand.
- Re-test on a schedule. Watch for alerts so a change in placement is caught early. Folderly monitors placement with deliverability alerts. Deliverability moves as your list, sending tools and DNS change.
Five tools for testing, checking and warming up
| Tool | What it does | Stands out for |
|---|---|---|
| GlockApps | Tests where an email lands across inboxes, scores it against spam filters, checks the sending IP against blocklists and verifies SPF, DKIM and DMARC | Inbox placement tests before you send |
| Mail-Tester | Scores a test email you send to it, with SPF, DKIM, DMARC and BIMI checkers and a DNS record lookup | Quick checks on a single message and its records |
| MailReach | Warms up mailboxes to help keep email out of spam and runs a spam test, with SPF and DKIM checkers | Warm-up for new mailboxes alongside the spam test |
| Folderly | Checks inbox placement, sending IPs and DNS, monitors placement with alerts, and has a DMARC monitoring tool | Ongoing placement monitoring for B2B outbound |
| Warmup Inbox | Warms mailboxes and offers a placement test, SPF and DMARC generators, a blacklist checker and a spam checker | A placement test its page says needs no signup (as of Nov 2026) |
For a longer comparison, there's a full roundup of email deliverability tools for testing and monitoring. And if you want something to tick through after a fix, the deliverability checklist is built for exactly that.
My advice: treat deliverability as a routine
Deliverability isn't something you fix once. It shifts every time you add a tool, change a DNS record or grow your list. Make two things routine: an SPF check whenever a new service starts sending as your domain, and a placement test on a regular schedule. That pair catches most problems while they're still small, long before your best campaign lands in spam.
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Frequently asked questions
What is the first thing to check when emails go to spam?
Authentication. Check that SPF, DKIM and DMARC records exist and pass for your sending domain, using any of the checkers in the table above.
What does an inbox placement test tell me?
GlockApps describes it as finding out where your email lands across Gmail, Outlook, Yahoo and other providers before you send.
Do I need to warm up a new mailbox?
MailReach and Warmup Inbox both offer warm-up for this reason: to build sending history gradually so a new mailbox doesn't look like a spam source.
What is an SPF permerror?
It's the result RFC 7208 requires when a record can't be interpreted correctly, which includes going over the limit of 10 DNS lookups.
How often should I test deliverability?
Regularly, and after every change to your sending tools or DNS. Placement can shift as your list and tools change, so monitoring works best as a repeated habit rather than a one-off check.
