I work for an e-mail newsletter company. Each day, I edit and publish newsletters that reach tens of thousands of people. Each day, I have a headline that serves as the subject line or, as in the Slate* e-mail above, enter a custom subject line. It’s a lot of pressure, and it should feel that way.
You cannot mess up a subject line — not with a spelling error like that. It’s one letter, but that’s all it takes. Most importantly, you only get one chance to win your audience, whether it’s headline or subject line. Also, spell-check. Use it. It’s better than nothing.
*Slate is awesome, and well-edited, and their newsletters have been a staple of my day for years. So I’m not bashing them specifically; I’m just shocked that they were my example.