Email forwarding anomaly

Cardew

I have a rarely used old Hotmail account.  (hotmail.com)

I received an email from another hotmail.com account. This was from a person with very little computer knowledge who lives in Europe.

Anyway I decided to forward this email to another email address I use. When I typed in the first letter of my other email address a list appeared of all HIS(the sender's) names and email addresses. I typed in several other letters e.g. A - B - C and each time a list of names and email addresses corresponding to that letter appeared; so I have access to his complete email address book.

To check I forwarded another email on this hotmail account, from a different person, to my other email address and a list of my MY names addresses appeared i.e. normal operation

Has anyone any ideas please - he uses Windows 10