This IRC operator only command lets you look up an email address to see if it is in use with NickServ and if so, which account(s) are registered with it. This command does not accept wild cards, therefore you must use a complete email address to return the correct results. To see if [email protected].com has registered any accounts, run this command:
/msg NickServ getemail [email protected]
Note that this command is quite noisy in that it will generate warnings to other IRC operators and leave messages about each and every use in the log file. It is therefore recommended you only use this command if absolutely necessary.