Try this on your Mac
To manage addresses in your Previous Recipients list:
To select addresses that are next to each other (contiguous) in the list, hold down Shift as you click the first and last addresses. To select addresses that are not next to each other in the list, hold down Command as you click each address.
- Choose Window > Previous Recipients.
- In the Previous Recipients window, do any of the following:
- To sort the list by name, email address, or the date the address was last used, click a column header.
- To search for a name in the list, enter the name in the search field.
- To view the card for a recipient who is already in your address book (a card icon precedes the recipient’s name), double-click the name. Address Book opens and displays the card.
- To add one or more selected recipients to your address book, click “Add to Address Book.”
- To remove one or more selected recipients from the list, click Remove From List.
Removing the recipient is useful if you don’t intend to use the address again or if you previously mistyped an address and want Mail to stop suggesting it as a matching address.
You can remove a recipient from the Previous Recipients list while reading a message you’ve received or writing a message. Click the arrow in a Smart Address and choose “Remove from Previous Recipients List” from the pop-up menu.
Update I just found this post from Tim Spracklen Super Moderator from 07/07/2011
Do you want the bad news or the very little good news......:(
The bad news....
The iPad keeps a separate, internal, 'recently used email address' list that is completely separate from, and not in any way related to, your contacts list. The Mac has the same list but, on the Mac, there's a way to edit/delete this list. Sadly and incredibly, on the iPad, no such facility exists - there is simply no way to delete this list.....
OK - the partial good news....this list is of 'limited length' and old email addresses eventually 'drop off the end' of the list...
Many Members have complained about this; maybe Apple read iPad Forum and hopefully they'll eventually do something about it.
PS it's raining cats and dogs here too..but that's not unusual for Scotland..