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[am]king ~/a/tm$ cat emails
user01 user01@emailaddress.com
...
user01
user02
user03

[am]king ~/a/tm$ sort -k1 emails > emails.sort
[am]king ~/a/tm$ sort -k1 names > names.sort

[am]king ~/a/tm$ join -v1 -o0 emails.sort names.sort
user04