How do I remove incorrect email adresses?

You can't. People have been asking about this since day one. There is no way to remove them. If you continue to ignore them, they are supposed to go down on the list of suggested addresses and I have read that they eventually disappear from that list. Frankly, I have seen no evidence of that yet in the 16 months that I have had my iPad.

Keep suggesting to Apple that they add this feature and hopefully they will.

http://www.apple.com/feedback/ipad.html

This is how to easily do it.

Mail (Yosemite): Manage email addresses in the Previous Recipients list

One thing you could try is go to your contact list and locate the "person" you were sending it to. There may or may not be there name in your list. If so, click on them and look at the e-mail address list. If it is there, edit it to the correct spelling. Once you go back to the e-mail you want to send out, you should see the incorrect e-mail address with an "i" icon with a circle around it. Click on it and you can delete the incorrect e-mail address permanently. You should also see the correct e-mail address closer to the top. Hope this helps!

when you see the incorrect address click the i and one of the options is remove from recents.

This is one of the most idiotic design errors Apple has made. There should be a way to clear the cache of old email addresses. So much time and energy has been spent on this board with this issue and Apple still doesn't do anything about it on the iPhone and iPad. Apple mail DOES allow users to delete old addresses.

On my iPad unused email addresses that are not in Contact will eventually go away.