InboxKit vs Mailforge: Google Workspace Mailboxes vs SMTP Relays

InboxKit vs Mailforge

Feature	InboxKit	Mailforge
Pricing
Price per Mailbox	$3.10-$2.50 by tier Professional $31/mo (10 slots), Agency $81/mo (30 slots), Enterprise $250/mo (100 slots). GWS, M365, Azure. Warmup +$3/mailbox add-on.	$2-$3/mo SMTP shared IP mailboxes
Cost for 200 Mailboxes	$500/mo 200 x $2.50 with deliverability tools included	~$484/mo Approximately $484/mo for 200 mailboxes
Infrastructure
Mailbox Types	GWS + M365 + Azure Multiple provider options	SMTP only Shared IP SMTP mailboxes only
IP Infrastructure	US IPs US-based IPs across all mailbox types	Shared IPs Shared IP pools across all mailboxes
DNS Automation	Full auto SPF, DKIM, DMARC, MX all automated	Yes + bulk updates Automated DNS plus bulk DNS update tools
Deliverability
Built-in Warmup	Yes Warmup built into the platform	No Must use external warmup
Inbox Placement Testing	Yes Built-in placement testing	No Not available
Features
Domain Masking	Not specified Not mentioned on the website	Yes SSL and domain masking available
Multiple Workspaces	Dedicated panel per domain Each domain gets its own panel	Yes Support for multiple workspaces
Bulk Domain Transferring	Not specified Not mentioned	Yes Domain transferring supported

How receiving servers actually treat the two mailbox types

When a Google Workspace mailbox sends an email, the receiving server sees traffic originating from gmail.com's sending infrastructure. The SPF record passes against a Google-owned IP range. DKIM is signed by Google's keys.

The MX record on the sending domain points to Google. All of these signals tell the receiving ESP "this is a real Google mailbox, evaluate under Google-class rules." When an SMTP relay sends an email, the receiving server sees traffic from a relay IP. SPF, DKIM, and DMARC all still pass, but they pass against the relay provider's infrastructure, not against an ESP-recognized sender.

Receiving servers evaluate this traffic under relay-class rules, which include stricter spam scoring and tighter rate limits to known relay IP ranges. This is not a bug in either system; it is how the email protocol layer of trust signals works in 2026. A well-configured SMTP relay can still get strong inbox placement, particularly at moderate volumes and with careful warmup.

But the relay-class evaluation is the headwind that the relay sender is always pushing against, while the Google Workspace sender starts with a tailwind.

Where the actual cost lands once you fill in the gaps

A naive cost comparison favors Mailforge on raw per-mailbox price. A complete one needs to fold in InboxKit's bundling structure: base tiers (Professional, Agency, Enterprise) include inbox placement testing, email verifier, and blacklist checker; Email Warmup is a $3/mailbox add-on on top of the tier base. At 200 mailboxes on InboxKit Enterprise: $250 base + 100 extra mailboxes at $2.50 = $500/mo for the mailbox layer.

Add the $3/mailbox warmup add-on if needed = $1,100/mo all-in. Without warmup it stays at $500. At 200 mailboxes on Mailforge: roughly $484/mo for the mailbox layer.

Add a standalone warmup tool ($30-50/mo), a placement testing tool ($30-50/mo), and email verification credits ($20-100/mo). The bundled Mailforge equivalent of InboxKit's non-warmup tooling lands at $564-$684/mo before warmup. Add a warmup tool and Mailforge's real bill approaches or exceeds InboxKit's with-warmup price.

For teams that already run those tools elsewhere, the bundled InboxKit value is less compelling because they are double-paying anyway. For teams starting fresh, InboxKit Agency or Enterprise often comes out cheaper than Mailforge plus the unbundled tooling stack.

When the mailbox-type difference becomes operationally meaningful

For most cold email teams below 25 active mailboxes, the GWS-vs-SMTP trust differential is small enough that careful warmup and clean sequences mask it. Both providers get acceptable inbox placement, and the choice can reasonably hinge on price preference and Salesforge ecosystem fit. Between 25 and 75 mailboxes, the trust differential becomes more visible.

Gmail and Yahoo start applying stricter scoring to relay traffic at higher volume, while GWS mailboxes maintain steady placement. Teams in this band who pick Mailforge over InboxKit often see placement decay after 60 to 90 days that they would not have seen on InboxKit. Above 75 mailboxes, the mailbox-type difference is large enough that teams who optimize for placement quality move to InboxKit (or to dedicated-IP SMTP via Infraforge for SMTP-specific use cases) regardless of price.

The savings from cheaper SMTP no longer compensate for the deliverability headwind.

Salesforge ecosystem fit and the structural lock-in question

Mailforge is the entry tier of the Salesforge infrastructure stack. Teams already using Salesforge sequences, Salesforge's sender tool, or other Salesforge products benefit from the in-account consolidation: one login, one billing line, one masterbox. The fit is genuine.

InboxKit operates as a more provider-agnostic infrastructure layer. It connects to SendKit, Instantly, Smartlead, and other sender tools without requiring you to standardize on a specific sequencer brand. For teams whose sender tool choice is independent of their mailbox provider, this independence is valuable.

The lock-in implication: Mailforge is the right choice if you are committed to Salesforge as a vendor family. InboxKit is the right choice if you want to keep your mailbox-provider decision separable from your sender-tool decision. This is a multi-year stack decision, not a per-purchase decision.