This can happen when using a browser addressing your email and if you use an email program like Outlook or Mac Mail.
This feature is generally called "auto complete". You start typing the name and a list of similar names popup to choose from.
Sometimes the list contains old names and email addresses or sometimes the list is automatically generated from incoming emails including spam that you would never want to send an email too.
Deleting names from the list will vary depending on the browser/email account, or Outlook, or someother email client.
If you use GMail you need to select Contacts on the left side,
- Log into your Gmail account
- Click the down triangle next to Gmail
- Select Contacts from the drop down menu
- Enter in the contact’s name or email in the search bar and select their record.
- Click in the contact’s email address field. Once you click in the textbox, a trash icon shows to the right. You can either edit the address and click Save Now on the top right or click the trash can icon to delete.
- If you no longer want the contact record, you can click More and select Delete contact.
If you use Outlook, it is a lot easier to remove the item from the auto-complte list. Open a new email and start typing a name, a list will appear and to the right of each name is an X. Simply choose the X and that name will not appear the next time.
Other email systems other than GMail, and other email clients other than Outlook will have different methods. Search the internet or the client's help for the specific instructions.