The m CLI isn't available in Docker containers. Replace exec.Command("m", "mail", "send")
with direct SMTP using crypto/tls + net/smtp (implicit TLS on port 465).
Env vars: SMTP_HOST, SMTP_PORT, SMTP_USER, SMTP_PASS, MAIL_FROM
Gracefully skips sending if SMTP is not configured.
Note: mgmt@msbls.de rejected by Hostinger as not owned by mail@msbls.de.
Default from address set to mail@msbls.de until alias is created.