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.

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Graham Mayor - MS MVP (Word) (2002-2019)
Visit my web site for more programming tips and ready made processes www.gmayor.com