Yours is actually a very common problem. Unfortunately, it’s a problem that can have several different causes, because auto-fill can come from several different places.
Your web browser
Let’s start with the most common one: your web browser. Very often what people believe is their web-based email automatically filling in something is actually their browser – Internet Explorer, Firefox, Chrome and the like – doing the work. By that I mean the browser is simply watching for fields that you type stuff into, and then remembers what you’ve typed in those fields. It has nothing to do with email; it just shows up there as well as in other places.
The fact that it’s happening in your email program is completely immaterial; it’s just remembering: hey, there’s this field called “To”; it was on this page; this is what I remember it having been used for in the past.
Fortunately, you can deal with this one easily by simply clearing your browser history. How depends on your browser. In Internet Explorer, click the gear icon over on the right, then Internet Options. Then on the General tab, click on Delete under browsing history. Make sure that something called “form data” is checked, and then click Delete.
It’s this form data that your browser is remembering for you when you’re typing in things like your email address or other fields.
Your address book
When it comes to web-based email, sometimes bad email addresses like this can in fact make their way into your contact list or address book. It’s particularly true if you’ve enabled any kind of an automatic address book addition feature, or if this feature is active by default. This would add the email address of anyone you send an email to automatically to your address book.
So, you mistakenly send to this bad email address once and poof, it’s in your address book. The email program or site then tries to be helpful when you’re typing email addresses by recommending entries from your address book; of which this bad address happens to be one.
The solution here is also fairly easy. Find the entry in your address book and delete it. You might also want to consider turning off that “automatic add” feature, if in fact there is one, and you can turn it off.
Your email program
Now, some email programs (Outlook comes to mind) and web email interfaces keep a list of automatically remembered entries, separate from the address book. In cases like this, the first two solutions may not work. Here’s what I’d have you try:
Start typing an email address into the “To” field. As soon as you see the bad email address being recommended as one of the potential candidates, use the down arrow key to scroll through the list of suggestions. Once you highlight the bad entry, type the delete key. In many cases, this will actually remove the entry not only from the current list of suggestions but from future suggestions as well. You may even find that as you mouse over the drop-down list the highlighted item will have an “x” to explicitly allow you to delete it.
As you can see, really this is all about the many ways your computer tries to help. Sometimes it tries just a little too hard. Hopefully, I’ve helped a little.