Open a personal inbox and most subject lines from real people are written in sentence case — it's how humans naturally type. That's part of why marketers increasingly favor sentence case: it reads as personal rather than promotional.

What sentence case signals

A subject line like "your invoice is ready" feels like a note from a colleague. The same message in Title Case — "Your Invoice Is Ready" — reads noticeably more corporate, and ALL CAPS often triggers spam filters or simply feels like shouting.

When Title Case still makes sense

• Formal B2B communications where a polished tone matters
• Brand-driven campaigns with a consistent style guide
• Subject lines that double as headlines in a newsletter archive

Test both versions quickly

Write your subject line once, then use a case converter to generate the Title Case and sentence case versions side by side. Running an A/B test between the two over a few sends will tell you far more about your specific audience than any general rule.