Lets say, for example, we have 3 MX record for domain @example.com. Assume that the hostnames below are mail server directly.
Pref Hostname IP Address
5 mail-1.example.com 100.100.100.10
10 mail-2.example.com 100.100.100.20
15 mail-3.example.com 100.100.100.30
If a sender wants to send email to firstname.lastname@example.org and the mail-1.example.com doesn't have account email@example.com, but the mail-2.example.com have the account for firstname.lastname@example.org, will the email sending fail? Or will the message send to the mail-2.example.com after mail-1.example.com told that it doesn't have for such user?