set mail-format { from: postmaster@mydomain.com }

set mailserver
   smtp.sparkpostmail.com
   port 587
   username "SMTP_Injection"
   password "mypassword"
   using tlsv1
 with timeout 30 seconds
 using hostname "mydomain.com"

set alert alerts@mydomain.com

Sendmail: 550 5.7.1 Unconfigured Sending Domain <localhost>

swaks -server smtp.sparkpostmail.com:587 -tls --auth-user SMTP_Injection --auth-password mypassword --to alerts@mydomain.com --from postmaster@mydomain.com