In German you can use “Guten Tag” as greeting when you see meet someone. When writing mail, you can also use it as greeting even if it is not that common, to use “Guten Tag” in written conversations.
BUT: When you use “Guten Tag” at the END it’s like a “Fuck off!” (regardless of written or in verbal communication) without being an insult.
I'd do the the opposite.
I generally use the default "cordially". But if someone pissed me off I'd use something overly formal and polite like "amicably thanking you for your eventual reply over the matter; if it pleases you"
The only time I use regards in a work email is me taking a subtle dig and dropping a WSB reference.
I appreciate you reaching out. If you would please refer to my previous email where I wrote out step by step instructions.it should answer all your questions. If you need further clarification, please feel free to reach out.