Check Subject Line: Tools, Tests, DNS

How do you check a subject line before sending?

A subject line check isn't one test. Each pass catches a different kind of mistake. A wording analyzer looks at the text on its own. A test send shows what a real mailbox does with the message. A header check and a DNS check tell you whether receiving servers can authenticate the domain behind the From address. Skip one and every error of that kind gets through.

1. 1 Wording pass: paste the subject line into a subject line tester. Note what it flags, such as all caps, stacked punctuation or phrases that read like an ad. Then decide for yourself whether each flag matters in a one-to-one B2B email.
2. 2 Render pass: send the finished message from your real sending setup to test inboxes you control, with at least one Gmail and one Outlook mailbox. Check where the subject gets cut off on desktop and mobile, what preview text appears next to it, and whether every merge field filled in.
3. 3 Header pass: in Gmail, open the test message, choose Show original and read the SPF, DKIM and DMARC results. In Outlook, open the message headers. If one of them fails, the problem is the domain, and rewording won't fix it.
4. 4 DNS pass: look up the sending domain's MX and SPF records (see the two sections below). Make sure they match the services you actually send and receive through.

Once the subject line clears all four passes, the next question is whether it's any good. For wording you can copy by situation, see our guide to cold email subject lines and follow-ups.

What does a subject line tester check, and what can't it see?

A subject line tester scores the text you paste in and nothing else. Most of them look at length, word choice, capitals, punctuation and emoji, then give you a score or a list of flags. That catches the obvious problems, like a subject typed in caps by accident or one stuffed with words that read like an ad.

A tester can't see your sending domain, so it can't tell you whether SPF, DKIM and DMARC pass. It doesn't know your domain's reputation with Gmail or Microsoft. It can't see a merge field, such as a first name, coming out empty for some recipients. And it can't tell whether the subject matches what the body says. Receiving servers give a lot of weight to authentication and sender reputation, so a high wording score on a domain that fails DMARC tells you very little. If your test messages already land in spam, start with our guide on why emails go to spam, not with a new subject line.

Most scoring models are also built for marketing newsletters. A short, plain, lowercase cold email subject can score low on an analyzer and still be the right choice for a one-to-one message.

Which free subject line tester should you use?

Several of the pages ranking for this search are subject line testers that run in a browser. The table compares them on the same points, using only what each tool says about itself in its page title in Google's results. We haven't added features we can't source. Check each tool's own page for its current scoring criteria before you rely on it.

Tool	How the page describes itself	Stated focus	What it cannot check
CoSchedule Email Subject Line Tester	"The #1 Free Email Subject Line Analyzer"	Free analysis of email subject line wording	Your domain's authentication, reputation or how merge fields render
VBOUT Email Subject Line Checker	"Email Subject Line Checker"	Checking email subject lines	Your domain's authentication, reputation or how merge fields render
ToolSpaceHub Email Subject Line Tester	"Email Subject Line Tester & Score Tool"	Giving a subject line a score	Your domain's authentication, reputation or how merge fields render
Supersend Subject Line Testing Tool	"Free Subject Line Testing Tool for Cold Email"	Free testing aimed at cold email	Your domain's authentication, reputation or how merge fields render

The deciding factor is fit. For cold outreach, a tester that says it's built for cold email fits better than one tuned for newsletters. Whichever you pick, treat its score as a second opinion on the wording, not a verdict on whether the message will arrive.

Which subject line checks do you have to do by hand?

Some of the mistakes that do the most damage in cold outreach are invisible to an analyzer. Check these yourself for every new sequence:

• Empty merge fields: if a contact has no first name on record, a subject like "{{first_name}}, quick question" goes out starting with a stray comma. Set a fallback value or fix the data before you send.
• Fake "Re:" or "Fwd:": adding either to a first-touch email makes it look like a reply to a conversation that never happened. It won't thread either, because threading depends on the In-Reply-To and References headers, not on the subject text.
• Deceptive subjects: the US CAN-SPAM Act prohibits subject headings likely to mislead the recipient about what the message contains. Make sure the subject matches the body.
• Non-ASCII characters: emoji and accented letters in a header have to be encoded into a special header format, and email clients decode it differently. Check the test send, not the editor preview.
• Preview text: right after the subject, the inbox shows the first line of the body or the preview text you set. A subject that repeats that first line wastes the space.

For how first-touch and follow-up subjects should differ, see our cold email subject line examples and rules and our follow-up email subject line examples.

How do you run a Google MX records check?

MX records tell other servers where to deliver mail for your domain. They matter for outreach because replies have to reach you, and because a sending domain with no working mailbox behind it looks odd to receiving servers. There are two ways to check them.

1. 1 Google's Admin Toolbox has a Check MX tool. Enter your domain and it compares the domain's mail records with what Google Workspace expects, then lists any problems.
2. 2 From a terminal, run: dig MX yourdomain.com +short. The output lists each mail server with its priority number. Lower numbers get higher priority.

Google's current Workspace setup documentation uses a single MX record pointing to smtp.google.com. Older setups list several aspmx hostnames, and Google still accepts those. Problems usually come from MX records left over from a previous mail host, which split incoming mail between two providers. Our page on Google Workspace DNS records for MX, SPF and DKIM covers the full record set.

How do you check an SPF record for Google?

SPF is a TXT record on your domain that lists which servers may send mail for it. To check it, run dig TXT yourdomain.com +short and find the line that starts with v=spf1. Google's Admin Toolbox Check MX tool also reports SPF problems, and so does our SPF, DKIM and DMARC checker.

Google's Workspace documentation tells you to include _spf.google.com in that record. If you also send through Brevo or another service, add those senders to the same record. Most failures come from two rules in the SPF standard published by the IETF. First, a domain may publish only one v=spf1 record. A second, separate record causes a permanent error, which receivers treat as an SPF failure. Second, the standard caps how many DNS lookups an SPF check may trigger, and every include counts toward that cap. Going over it also causes a permanent error, so remove includes for services you no longer use. Then send a test message and confirm that spf=pass appears in the Authentication-Results header.

How does EmailFlow OS handle the message behind the subject line?

EmailFlow OS isn't a subject line tester, so it sits outside the comparison above. It's a EmailFlow OS product for personalised video email in B2B cold outreach, and it sends under your own domain, not a shared platform address. You record one video, upload it to EmailFlow OS, connect your Brevo account and sending domain, and import your prospect list into EmailFlow OS. EmailFlow OS then personalises that one recording for each recipient, not each company, and sends every version from your own inbox, through your own Brevo account and your own sending domain.

Because the mail goes out under your domain, the MX, SPF and DKIM checks above are required. They determine how receiving servers authenticate every video email you send. The receiving server still decides whether a message reaches the inbox. The setup is described on the EmailFlow OS personalised video email page.

Questions people ask

? Can a subject line alone send an email to spam?

It can contribute, but it's rarely the only cause. Spam filters weigh authentication (SPF, DKIM, DMARC), the sending domain's reputation, complaints and content together. A message that fails DMARC can land in spam whatever the subject says. On a well-authenticated domain with few complaints, a clean subject usually isn't what decides it. Check the Authentication-Results header before you rewrite the wording.

? Is a subject line tester score reliable for cold email?

It's a reliable check on wording, not on delivery. A tester looks only at the text: length, capitals, punctuation and word choice. It can't see your domain, your reputation or your merge data. Many scoring models are built for newsletters, so a short, plain, lowercase subject can score low and still suit a one-to-one B2B message.

? What is the difference between Check MX and dig?

Check MX in Google's Admin Toolbox is a web form. It compares a domain's mail records with what Google Workspace expects and explains any problems in plain language. dig is a command-line DNS lookup that returns the raw records, such as each MX host with its priority, without interpreting them. Use Check MX for a quick diagnosis and dig to see exactly what's published.

? Why does my SPF check say permerror?

A permanent error (permerror) means receivers couldn't evaluate your SPF record at all, and they treat it as a failure. There are two common causes. One is more than one v=spf1 TXT record on the same domain. The other is a record whose includes go over the lookup cap in the SPF standard. Merge everything into one record and remove includes for services you no longer send through.