Not necessarily, "checking if the user exists on the server" is ambiguous. You could check /etc/passwd, but the mail system may use virtual users for "local" d

Not necessarily, "checking if the user exists on the server" is ambiguous. You could check /etc/passwd, but the mail system may use virtual users for "local" delivery, where "local" is defined in this case as not requiring a domain portion. The only way to even check if a user exists even locally is to try to send it mail.