Domain reputation is one of the biggest factors in cold email deliverability. This covers how to pick and configure domains for Maildoso campaigns, including which TLDs to avoid.

Configuring SMTP mailboxes effectively is crucial for maintaining domain reputation, optimizing deliverability, and scaling email campaigns. Below are the best practices and guidelines for setting up SMTP mailboxes per domain.

Proper warmup and sending practices are essential for maintaining deliverability:

Yes, eight mailboxes per domain is within the recommended range of 4–8.

For 20 mailboxes, use 3–5 domains, keeping 4–8 mailboxes per domain. By following these guidelines, you can optimize your SMTP mailbox configuration for better deliverability and scalability.

Stick to established TLDs for cold email: .com, .info, .co, and .one are all safe choices.

Avoid .online, .site, .xyz, .space, and .website — these TLDs have a track record of being suspended by their central registry when used for bulk cold email, which takes the domain (and any mailboxes on it) down with no warning.

.click and .top can be kept on a case-by-case basis rather than being recommended by default — ask support if you're specifically considering one of these. .sbs and .cfd are currently above what we consider an acceptable risk level and should be avoided.

If you already have mailboxes on a domain with a risky TLD, you don't need to panic or pause sending — but if that domain does get suspended, replacing it is the fix; see Domain replacement. If the suspension shows in your dashboard as MANUALLY DISABLED, see My domain shows "MANUALLY DISABLED" — can it be reactivated?

Before using any domain for a campaign, it's worth checking it isn't already listed on a major blacklist. Aged domains (registered and DNS-active for a while) generally have a head start on reputation compared to a domain created the same day you start sending. You don't need to manually verify this before every send — Maildoso monitors reputation on your domains — but it's good to keep in mind when choosing which domain to put where. If a domain does get flagged, see Domains flagged as blacklisted: how we help, and where refunds fit in for how we handle it.

If your primary company website domain isn't already connected to Maildoso, don't connect it for cold email — use a separate domain dedicated to campaigns instead, so a deliverability issue on a sending domain never touches your main site's reputation. If your primary domain is already connected to Maildoso and you want that separation now, see Can I stop using Maildoso's DNS or move my domain to another DNS provider? to move it back to your own DNS.

We recommend 4 to 8 SMTP mailboxes per domain as a default, and up to 50 mailboxes on a single domain is a supported, tested configuration — we've tested that density extensively and it performs the same as a lower-density setup. See Setting up domains and mailboxes for the full breakdown, including Google Workspace limits.

The reason to spread mailboxes across more domains isn't placement — it's blast radius: a domain-level problem takes every mailbox on that domain down at once, so fewer mailboxes per domain means less exposure to any single incident. Density does change domain-level volume, though, since sending limits are per mailbox — 50 mailboxes at 15 campaign emails/day is up to 750/day from one domain. See What's the daily email limit per email account?