To edit the drop down list you need
http://www.nirsoft.net/utils/outlook_nk2_edit.html
The list has nothing whatsoever to do with your e-mail account. It is an Outlook centred database.
If your account has been hijacked and is sending out unwanted e-mails that is another issue entirely. You don't supply any information about what is happening, but you could start by downloading and running the free version of MalwareBytes -
https://www.malwarebytes.org/
Also run an on-line antivirus checker.