Does using all caps matter?
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You've seen it before. A subject line screaming "HUGE SALE ENDS TONIGHT!!!" lands in your inbox and your first instinct is to either ignore it or hit spam. Filters feel the same way.
All caps does matter for deliverability, and the impact scales with how much you use it. One capitalized word for emphasis is usually fine. A fully capitalized subject line is a different story. Filters like SpamAssassin calculate the ratio of uppercase to lowercase characters and add score points when that ratio tips too far. Enough points and your message gets filtered before anyone even sees it.
The subject line is where this matters most. Body copy in all caps compounds the problem, but the subject is the first place filters look and the first thing humans see. A subject like "OPEN NOW FREE GIFT INSIDE" stacks multiple signals at once: aggressive caps, urgency language, and a freebie offer. Each one adds points. Together they can easily cross the threshold.
There's also a human loop that people miss. Even if an all-caps email makes it through the filter, recipients often report it as spam because it feels aggressive. Those spam reports feed back into your sender reputation and make the next campaign harder to deliver.
The fix is simple. Use capitalization the way you'd use it in a conversation. One word, occasionally, for real emphasis. Everything else in normal sentence case. Your subject line doesn't need to shout to get opened. It needs to be relevant.
Want to test how your subject line scores before you send? Try our free subject line tester and see what flags come up.
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