Is it better email etiquette to send one email spanning multiple topics, or to clutter someone’s inbox with multiple, single-topic emails? Simple answer: One subject per email, please.
It is better to receive three emails regarding three different subjects than one email containing three subjects. In other words, please limit one topic per email.
In today’s world of automation, emails tend to be sorted by subject, often automatically, and if you include multiple topics in one email, those subsequent topics are prone to be misplaced, misfiled, and/or overlooked. Many people file emails in specific folders for future reference, and having an email with multiple topics can easily foil a simple filing system.
A common example is when you use a tool like Basecamp (an online project management/communication software). If you reply to a Basecamp email, the software adds it to the original message thread. When you cover multiple topics in that same email, it will be difficult for other Basecamp users to find those other topics later.