Inbox and sending infrastructure
providers.
Inboxes are the largest recurring line in a cold email infrastructure bill. A GrowthPal engagement runs roughly 110 inboxes across 40 to 50 domains, at two to three inboxes per domain, and that count is arithmetic derived from target volume rather than a preference.
This page covers what these providers supply, the arithmetic behind the inbox count, and the four providers in this category.
Where the inbox count comes from
Target sending volume sets the number of inboxes, because a single mailbox can only send so much before the pattern looks wrong. Inboxes then set the number of domains at two to three per domain, so that one bad domain costs a slice of capacity rather than the whole campaign. The inbox calculator performs the conversion, and the cost page shows the resulting bill at 110 inboxes.
Bought in bulk, not one at a time
At this scale the provider is chosen on bulk provisioning, DNS handling and per-mailbox cost rather than on the features an individual user would care about. The domains themselves come from a registrar; see domain registrars.
The inbox infrastructure in the directory
Each take below is GrowthPal's own, written from the operator's side of the desk. They are opinions about fit, not reviews, and they carry no rating. The directory is built from tools GrowthPal uses. No listing here is sponsored, affiliated or paid for, and there is no commercial relationship behind any entry.
| Tool | GrowthPal's take |
|---|---|
| InboxKit Review | Inbox infrastructure with quick provisioning and dashboards that make 100 mailboxes manageable. A GrowthPal partner for the infrastructure side. |
| Mailforge Review | Spin up sending domains and inboxes in bulk without living inside admin consoles. DNS handled, sanity preserved. |
| Premium Inboxes Review | Google and Microsoft inboxes built specifically for cold email scale, provisioned fast and priced for volume. We partner with them, and we'd recommend them anyway. |
| Zapmail Review | Google and Microsoft inboxes provisioned at scale, at prices that make proper volume viable. |
Frequently asked questions
How many inboxes does a campaign need?
The inbox count follows from target sending volume, because a single mailbox can only send so much before the pattern looks wrong to a mailbox provider. The inbox calculator converts a volume into an inbox count and then into a domain count at two to three inboxes per domain. A GrowthPal engagement typically runs roughly 110 inboxes across 40 to 50 domains.
Why not send more from fewer inboxes?
Two reasons. Volume per mailbox is one of the signals mailbox providers watch, so pushing more through fewer inboxes makes the pattern look less like a person and more like a bulk sender. The second reason is fragility: when a domain has a bad week everything on it pauses together, so concentration turns a local problem into a campaign-wide one.
Is the infrastructure included if an agency runs the campaign?
In GrowthPal's case, yes by default. Domains, inboxes, warmup, data and verification are covered by the retainer, and a client who would rather hold the infrastructure in their own accounts can. Ownership does not change either way: everything transfers to the client at the end of an engagement. Other agencies differ, and it is the single biggest reason two quotes are not comparable.
Related
Sources
- Tool takes: the GrowthPal tool directory.
- GrowthPal's own figures: how the numbers on this site are counted.
Last reviewed . Vendor pricing and features are not published here and are not reviewed on this cadence.