Apple’s Mail for OS X has a feature whereby it stores your previous recipients so you can quickly address new emails to people you’ve already communicated with. This helps make emailing quicker, since you may need to email people who aren’t in your Contacts list.
In addition, your iCloud account syncs these previous recipients to your iOS devices (if you sync contacts with iCloud), so if you’ve emailed someone on your Mac, their address will be quickly available on your iPhone.
However, these previous recipients can get in the way, especially if you reply to a lot of people who you don’t want to keep as contacts. From time to time, I go through the list to cull those people I don’t plan to be emailing again, so when Mail suggests auto-completion for email addresses, there aren’t too many.
You can only do this in Mail on OS X; there’s no way to clear these addresses on iOS devices. In Mail, choose Window > Previous Recipients. (Sorry, I have to blur all the names and addresses…)
You’ll see names, email addresses, and the date you last sent an email to each person. You’ll also see icons in the loft column for those people who are in your Contacts.
Here’s how I cull them. I first click the leftmost column header to sort all those people in my Contacts together. I select them all then delete them; this doesn’t delete the contacts, but just their entries in the Previous Recipients list.
Next, I sort the remaining people in the list by name. For those I want to get rid of, I delete them by pressing the Delete key. For the ones I want to add to my contacts, I select a few at a time, then click Add to Contacts.
You might want to keep some of these names and email addresses in the Previous Recipients list; such as those with whom you’ve corresponded recently. To do this, click on the Last Used column header to sort them by date, and delete all of them that are, say, more than a few months old.
Think about clearing these addresses every few months, and you’ll have fewer auto-complete suggestions in Mail on the Mac, and on your iOS device.