Regards & Regard

Regard (singular noun) means a look, a gaze, an aspect; and by extension esteem or attention.
Regards (plural noun) means the wishes that express such esteem or respect.

Though grammatically either noun might work, in social convention only the plural noun is normally seen. It is an informal abbreviation of the phrase "with best regards" or similar phrases.

Regard is just a misspelling - which is probably why your boss didn't like it.

"Regards" is short for "my best regards" or similar. "My best regard" would not make sense as regard can not be a singular noun.

Imagine writing, "Yours incerely" at the end of a letter. It gives a bad impression.

The suitor says "My feelings for you are genuine."

Congratulations! The girl believes him.

The news is that they are to be married soon.

The king sends his regards.

Toward (AE) / Towards (BE) a Better Understanding.

The letter/sound "S" is very powerful in English. So, be mindful when you run into one. It could mean the difference between: HE and SHE. MR and MRS.

No wonder your BOSS doesn't want you to go around dropping "Ss". Next thing you know, you'll be calling the boss, BO, an anglicized form of beau, boyfriend.

Tongue-in-cheek? Yes. But, you should get the point. And that is what counts!