This type of attack is called as phishing emails.
Step-by-step explanation:
Normally any bank will not ask end user about login details such user id, password, pin number in email. So bob as think and before responding the mail. Moreover this type email to be considered as hacker email, who want to access the bob accounts and do access his account and steal money from bob accounts.
Luckily bob contacted bank to stop further hackers not to his accounts. Since it is grammar mistake bob understood it is hackers who asking bob user id and password.
Bob has prompted these type of mails are called phishing emails. Better to avoid it.