o365discover can easily check whether users exist or not.
$ o365discover -u [email protected]
[email protected]
$ o365discover -u [email protected] # nothing
$ o365discover -f emails.txt
o365discover can easily check whether users exist or not.
$ o365discover -u [email protected]
[email protected]
$ o365discover -u [email protected] # nothing
$ o365discover -f emails.txt