Yes, it is to avoid spam filters that look for certain phrases. By having an mass mailing program insert a random quoted word inside the offending phrase, or quote a random word in the phrase, that phrase doesn’t match what is being filtered and sometimes gets through. It is also why specific words like p3n1s and v1agra are spelled like they are.
But the OP’s examples are NOT randomly inserted, nor are they misspelled.
I think the simplest answer to the OP is that, unfortunately, relatively few people today understand the proper uses of punctuation. And one of the very most common errors that I see is for the quotes (single or double is irrelevant) to be used for emphasis. I really have no idea why people think that quotes are the equivalent of underlining, but they sure seem to.
It is true that these things are not available in email headers. And for the old fogies among us, it used to be the case that even the body of the email was plain text only.
I have a pet peeve about using quotes for emphasis, because quotes have an established pre-existing use, and it is to DE-emphasize something, and introduce a specifically sarcastic tone to the statement. Quotes are an alternative to the term “so-called”. Example: He was an “intellectual” does not mean that he was a truly great intellectual; it means that he was called an intellectual but truly was not.
I find it bizarre that one who is in a situation where underlining, bold-face, italics, and other formatting effects are unavailable would choose quotes for emphasis, when ALL CAPS provides that effect so much more easily.
I am not aware of any such point in time. All sorts of effects were available to those who write with a pen or pencil. Even typewriters were capable of underlining and bold-face. (Bold was created by retyping and adding more ink to what was already typed.) And, as above, using all upper-case has always been an option (except in Morse code, I suppose, which doesn’t have quotes anyway).
Correct, which is why I also said quoting words already in the phrase.
For example, if a mail server sets a filter for increase your manhood then it looks to block that exact set of characters, adding quotation marks means “increase” your manhood and increase your “manhood” make it past that filter but still remain human readable.