I'm in industry, but in higher ed (college), so it's a slightly unique part of industry when it comes to things like letters after people's names in their signatures. Many of the employees on campus are professors, who of course have “Dr.” in front of their name and “Ph. D.” after their name, and may have a few other things too. Upper administration, especially on the academic side, is likely to have letters after their names as well. So, in my last industry job, CPA would have been one of the very few people with letters after their name, but in this industry job, letters after your name is common, and is common in email signatures.
That being said…the first few months on the job, I had CPA in my signature. I'm the Controller, I was 24 when I started the job replacing someone who had just retired, and I wanted that extra bit of “clout” to give people internally – who largely had a string of letters after their name – a validation that I might know what I was doing. Then after a few months, I took it out, because I'd established my reputation on campus and it seemed kind of weird having it there. My boss (CFO/VP of Bus and Fin) isn't a CPA, and don't put MBA in his signature (which I'm glad of – Master's don't belong in signatures lol), and so without the precedence in my reporting structure, it felt a little weird. However, I'm considering putting it back now. I've had a few conversations via email with people referencing “I know a CPA who…”, and realized that CPA means more to them than 25-year-old Controller. I've heard, though, that if you have CPA on your email, then your license is more liable for anything you say, and even though I try to always do things correctly, I'm not sure if some vanity is worth risking my license for the emails that I send at the end of a 16-hour day when I might not be at the top of my game. So, I'm thinking about putting CPA back in my signature (and have also seen that it's common for many other people in my position at other colleges), but haven't done so yet.
Oh, when I got my CPA at my prior industry job where no one had letters after their name and I was in a more entry-level position, I didn't add it after my name. It didn't seem appropriate in that setting. That was largely due to being in a lower-level position, though.