When you go to type in a name in the To: section, if there is an X button on the right side in the suggestion list, then it can be deleted from the history of recipients via click on that button.
Otherwise, the unwanted contact must be deleted from your address book or the messages sent to this email address from the Sent folder.

You can also adjust your suggested recipients list by going to Menu > Settings > Mail > Compose > Email Address Suggestion.

Suggestions based on messages in Sent folders and Recipients history based on your use in eM Client can be turned off; suggestions based on your saved Contacts cannot be disabled.