There is a feature in Mac Mail that I found extremely useful.

When composing an email, Mac Mail was able to determine which account I was meant to be sending from based on who I am sending to.

For example:

Lets say we have 2 email accounts, account 1, 2 and 3

When I email customer 1, I always use account 2.
When I email customer 2 I always use account 1.

Mac Mail learns this pattern and auto swaps to the correct account when I add the recpient to the email.

In my case I use at least 8 different accounts and on many occasions I've sent from the wrong email account.