Configuring sending throttles

Throttling controls how fast your mail leaves: how many messages per hour go to each recipient domain, and how many connections you open at once. Steady, paced sending protects your reputation; bursts get you rate-limited or filtered.

Why throttle

Mailbox providers watch the rate at which you send, not just the volume. Sending a huge burst to Gmail or Outlook in a few minutes looks abusive and triggers temporary deferrals (you'll see 4.x.x errors). Throttling spreads the same volume out so it arrives at a rate providers are comfortable accepting, which is also exactly what you need while warming a new IP.

What you can control

Throttling rules are applied per sending IP and scoped per recipient domain, so you can pace Gmail differently from Outlook. Each rule sets:

• Messages per hour: the cap on how many messages that IP sends to the matched recipient domain(s) each hour.
• Concurrent connections: how many simultaneous SMTP connections the IP opens to that domain.

Matching recipient domains

Each rule lists the domains it applies to, one per line. There are three kinds of entry:

• A domain, such as gmail.com, matches that domain exactly. It doesn't cover subdomains or look-alikes.
• mx: and a host, such as mx:google.com, matches any domain whose mail servers all end in that host. This is how you reach companies that use Google Workspace or Microsoft 365 on their own domain names.
• * matches every other domain, so you can set a catch-all limit.

Templates & overrides

You manage throttling in the console under Virtual MTAs → Throttling:

• Throttling templates: reusable sets of rules (e.g. a conservative profile for warming, a higher one for established IPs). Assign a template to an IP and it inherits those rules.
• Per-IP overrides: override a template rule, or add a custom rule, for a single IP. For example, a tighter limit on one domain while you investigate an issue there.

An IP's effective rules are its template rules with any overrides applied on top.

You'll also see a few presets marked BlacklistGuard, such as the Default Template and the Conservative and Aggressive profiles. You can use them in your own rules and give them to your own IPs, but you can't edit or delete them. To change one, create your own template or profile with the settings you want.

Using throttles to warm up

Dedicated IPs you buy from us warm up automatically, so you don't need throttles for that. You'll see their warm-up listed as their throttling while it runs, and you can't change it.

For an IP you bring yourself, throttling is the mechanism behind a manual warm-up. Start it on a conservative template (low messages-per-hour), then raise the limits step by step over the following weeks as its reputation builds. See Warming up a dedicated IP for the full ramp.

Pair throttles with routing

Throttles decide how fast; routing rules decide which IP. Use them together: route a stream of mail to a specific IP, and throttle that IP for the destination, controlling both the path and the pace of your sending.