To extract user's email, you may search from table USR21 and ADR6

  1. SE16 > USR21 and enter user ID
  2. Copy the PERNR (personal number) 
  3. SE16 > ADR6 and paste the PERNR
  4. You will get a list of email addres
Alternatively, you may replace Table ADR6 with  PA0105 but this table only exist in HR system.