Is all-caps forbidden?

You've probably heard that writing in ALL CAPS will get your email flagged as spam. It's one of those "rules" that gets repeated so often it starts to feel like fact. The real answer is more nuanced than that.

All-caps is not forbidden. Spam filters don't have a binary rule that says "caps found, send to spam." Modern filters look at the full picture: your sender reputation, authentication setup, engagement history, and content patterns together. A single GREAT DEAL in a subject line isn't going to sink you.

That said, there are real reasons to use caps carefully.

The UX problem is bigger than the filter problem. When readers see a subject line like "FINAL HOURS BUY NOW LIMITED STOCK", it reads like shouting. People don't like being shouted at. Lower open rates and higher delete-without-opening rates follow, and those engagement signals do affect your deliverability over time. The filter didn't flag you. Your readers did, quietly, by ignoring you.

The pattern matters more than any single word. If your emails consistently use heavy caps alongside words that look promotional, exclamation points, and aggressive CTAs, the combination starts to look like a pattern spam filters recognize. It's never just one thing.

A good rule of thumb: caps work well for a single word or short phrase where you genuinely want emphasis (think "NEW" in a product launch or "LAST CHANCE" when you really mean it). Full sentences in caps, or multiple capped phrases in one subject line, start to feel aggressive to readers and potentially noisy to filters.

So don't avoid caps out of fear. Avoid them when they'd make your email harder or more unpleasant to read. That's the real test.

Want to check whether your subject lines are tripping any other content-level signals? Our Subject Line Tester is free and takes about 30 seconds.