Should we always use the exclamation mark in expressions like "thank you", "my pleasure", etc?

Solution 1:

The answer is no. Grammatically no and actually no.

Grammatitically, exclamation are to be used to exclaim a point. And in the above quote, the sentence doesn't seem exclaimed. Usually when something is exclaimed the speaker's pitch rises at the end of the sentence.

Actually, native English speaker rarely (nearly never) use exclamation marks. Usually, exclaimed sentences are short like "Thank you!" or "Help!".

Solution 2:

As a native English speaker whenever I receive an email with Thanks! At the end it reads in my head as a condescending or cynical expression of thanks.

I much prefer to see the use of the word Thanks ended with a full stop or nothing at all. In fact it seems more natural when reading it without one as I don't interpret the thanks as rising in pitch for any reason at all and thus not a cynical "Thanks!".