A sloppily written thank you letter can ruin your chance at getting the job, so always do a thorough check before hitting that send button. If fact, sometimes it is better to send the email to yourself, wait 15 minutes and then reread the email to make sure everything looks good.
Beyond grammar and spelling, make sure that:
1) Names, dates and email addresses are correct
2) The correct company is mentioned, especially if you’ve been interviewing at other places (I once received a thank you email that included the name of our company’s competitor)
3) Similar to the previous point, you also want to make sure you included the correct job position that you interviewed for