Does using ALL CAPS trigger spam filters?

Picture a subject line that reads: HUGE SALE THIS WEEKEND DON'T MISS OUT. Does it make you want to open the email, or does it feel like someone is yelling at you? Spam filters tend to agree with your gut on this one.

Yes, excessive ALL CAPS can hurt your deliverability. Spam filters look at a long list of signals when they decide where an email lands, and capitalization is one of them. A subject line written entirely in caps is a classic pattern associated with aggressive, low-quality sending, and filters learn from those patterns.

SpamAssassin (one of the most widely used filtering engines) has specific rules that check the ratio of uppercase to lowercase characters. When the uppercase percentage tips too high, it adds points to your spam score. On its own, a few caps won't kill you. But combine it with trigger words and other red flags, and you're stacking up points you don't want.

Here's a rough guide to what's fine versus what's risky:

• Fine: "Your order is CONFIRMED" or emphasizing a single word like "FREE shipping today"
• Risky: "HUGE DISCOUNT THIS WEEKEND ONLY" or any subject line where every word is capitalized
• Really risky: ALL CAPS subject line plus exclamation marks plus words like "act now" or "limited time offer"

Beyond filters, there's also the human side. ALL CAPS reads as shouting. Even if an email gets through, recipients often find it off-putting, and poor engagement over time will hurt your sender reputation more than any single subject line ever could.

The fix is simple. Use capitals for genuine emphasis on one or two words, max. Let the strength of your copy do the talking, not the shift key.

Want to test how your subject lines score before you send? Try our free Subject Line Tester to catch these kinds of issues ahead of time.