Formatting and layout are often overseen as tools of effective communication. We have to remember that text is just a convention of graphic symbols and how they fit together to transmit ideas. The layout of your text can help or hinder your communication.
Some design choices are already made for us by our writing conventions. We write - in English - from left to right, in lines formed by bottom-aligned characters and blank spaces defining words. Other design choices are made by the medium. Email clients usually have pre-defined line-height and line-breaking rules, and limited choices for font families.
Then you have the traditional rich-text formatting options - bold, italic, underline, font color, text-alignment, some indentation, and ordered or unordered list. These are useful, but the most powerful formatting tool for effective communication does not have a button for it. It is simpler than that; it is the blank line.
Yes, I am talking about paragraphs, but the simple line break isn't enough to visually separate paragraphs in emails. An extra hit on <enter> is necessary. All paragraphs should be separated by blank lines in emails.
Paragraphs in emails
The obvious fundament for placing the blank lines is to decide what forms a paragraph. Paragraphs in emails are not the same as paragraphs in essays or literary texts; they are much more granular and topical.
Emails are more akin to dialogues than storytelling. So blank lines are how you punctuate your speech to check if your reader is following. As a consequence, paragraphs are much shorter on emails than on essays. Usually, no more than two or three sentences.
Paragraphs also show emphasis. If you have an important question that you must receive an answer, it deserves its own paragraph. Isolate the one-sentence question between blank lines.
If it's crucial to get an answer, you can even boldly make it bold. A widespread miscommunication pattern is replying an email while ignoring questions that were asked in it. Prevent this by making very hard, even awkward for the interlocutor to ignore a question you are making.
Emails are often read diagonally, in a rush or reread by someone looking for a specific text snippet. Blank lines help people find what they want quickly.
Blank lines also help the reader to understand complex concepts. A blank line is a visual cue that you should be able to comprehend all that was written until that point. There will be no further sentence that will better explain that concept.
That is also a guideline to follow when writing your paragraphs. A paragraph should have a meaning by itself. If you a couple of sentences are not enough to convey meaning by themselves and need the clarification of a third sentence, then that third sentence should be in the same paragraph.