SMTP authentication vs IP allowlist: which should I choose?

Smarthost offers two authentication modes. Pick the one that fits your setup:

SMTP authentication (SASL) — recommended for most.

• A unique username and password per domain.
• Works from any IP, including dynamic or changing addresses.
• Credentials can be rotated freely if ever exposed.
• Constrains the sender domain, so one customer can't spoof another.

IP allowlist — simplest for fixed-IP servers.

• You give us your static sending IP(s) and we trust that source with no credentials.
• Easiest config for a datacentre server with a stable IP.
• Requires stable, non-dynamic IPs — not suitable for residential or NAT-pool addresses.

If in doubt, choose SMTP authentication: it's portable and rotatable, and works everywhere.