If you're using Apple Mail and are experiencing issues with failed attempts to send out a message because one or more of your friends' email address is borked, then your Previous Recipients may be to blame. Sometimes your Mac needs some general housekeeping.
It's one of those hidden gems in the Mail app that not many know about. Ok, so perhaps it's not much of a gem, but it is supposed to make life easier for you when it comes to composing and sending mail messages. You see, every time you send a mail message, the recipients — people you send the message To or CC or BCC — get saved into this handy-dandy section called the Previous Recipients window. The next time you go to email someone, you can just start typing a few letters of the person's name or email and the Previous Recipients List will serve up a history of all email addresses that match those letters regardless of if it's been stored in your Contacts (Address Book for those still on legacy systems). Unfortunately though, this same "feature" will also serve up any of the incorrect email addresses as well which in many cases, tired eyes may not catch and then all the fun starts. [sarcasm]
These email addresses congregate there like dustbunnies in heat and periodically just need to be swept up and sent away to never never land.
Like my 3-year old now says,
"But, WHY Mommy?"
Because I said so.
Actually, because, if you're having problems sending messages — and I've seen this too many times to count — (which is why I've decided to make it a Hand-Made Mac Tip) it's usually because you have several wrongly entered email addresses hiding in this list somewhere and it's just a matter of time before one of them bites you back.
How does this happen?
Two words: fat fingers.
Ok, one more word: cats.
It happens whenever you (or your cat) accidentally type an email address wrongly despite being absolutely certain that their email address was email@example.com
or, how about: firstname.lastname@example.org
Can you spot the mistakes? Does your cat care? If you answered no to either of those questions, then you've got issues.
Well, guess what. Mail servers can spot the difference but for some dumb reason, Mail collects and stores these boo boos in your Previous Recipients list despite this fact.
Here's how to clean it out and start fresh.
In Mail, select Previous Recipients from the Window menu. In the next window, select all of the addresses in this list and then click the Remove From List button. (Just do it, don't hesitate, be ruthless! Zap them all!)
Now this list will repopulate over time (rinse and repeat), but any badly entered email addresses will no longer cause problems with outgoing messages.
I hope this was helpful and that you feel all sparkly fresh now!