This isn’t specifically a Mac tip but it’s a useful trick nonetheless.
I have two main email accounts that I use for historical reasons: Gmail, and iCloud. Because iCloud email is so woefully poor, I try to send (and receive) all my mail using Gmail. However, some people send email to my iCloud address because they know no better, or because their email client fills it in for them automatically.
I set Gmail as the default for new mail but sometimes my mail client decides that, actually, I really want to send email using my iCloud address. Alternatively, when replying to email received by iCloud, I want to be able to reply using Gmail but in my haste often don’t realise which account I’m using.
THE SOLUTION: If your email client allows you to have different email signatures for each account, add a special signature for the account you DON’T want to use. Make the signature a warning, along the lines of “DON’T USE THIS EMAIL ADDRESS!!”. Then set the signature for all new mails, and replies, sent using that address.
In future if your email client randomly decides to use the new address, you’ll be able to see the error in no uncertain terms and switch to your preferred default address!