The deliverability tool stack most senders need has three layers, and the first one is free. Google Postmaster Tools, Microsoft SNDS, Yahoo Sender Hub, mail-tester, and MXToolbox cost nothing and give you authoritative data straight from the mailbox providers. Add a placement-testing tool like Glock
The deliverability tool stack most senders need has three layers, and the first one is free. Google Postmaster Tools, Microsoft SNDS, Yahoo Sender Hub, mail-tester, and MXToolbox cost nothing and give you authoritative data straight from the mailbox providers. Add a placement-testing tool like GlockApps and a warm-up tool, and you have covered about 90% of what an enterprise deliverability suite provides, at a fraction of the cost, for any operation under 500,000 emails a month.
This post organizes the tools by the job they do: layer 1 is free ISP data you should always have, layer 2 is placement testing you run during warm-up and before campaigns, layer 3 is warm-up and ongoing monitoring. Every price was checked in August 2026.
Email deliverability tools are software that helps a sender monitor and improve whether their email reaches the inbox. They fall into three layers: free ISP feedback tools that report complaint rates and authentication directly from providers, inbox placement testing tools that show folder placement across mailbox providers, and warm-up and monitoring tools that build and track sender reputation over time.
You need at least one tool from layer 1 at all times. Layers 2 and 3 scale with how much you send and how much is at stake.
Google Postmaster Tools. The only source of Gmail's own view of your sending: spam complaint rate, feedback loop, authentication results, delivery errors, and a compliance panel. The v2 version dropped the domain and IP reputation charts, so spam rate is now the headline metric, and the target is below 0.1%. See our Google Postmaster Tools guide.
Microsoft SNDS. Smart Network Data Services shows how Outlook.com, Hotmail, and Live rate your sending IPs, with green, yellow, and red filter codes and a complaint rate column. IP-based, so most useful on a dedicated IP. See our Microsoft SNDS guide.
Yahoo Sender Hub. Yahoo's equivalent, covering yahoo.com and AOL. See Yahoo sender requirements.
mail-tester. A free content and authentication score out of 10, checked against SpamAssassin. Use it as a pre-send content check. See spam score checker.
MXToolbox. Free blacklist lookups across roughly 80 lists, plus DNS and authentication checks. See email blacklist check.
GlockApps. The standard for pre-send placement testing. Sends your message to seed inboxes across Gmail, Outlook, Yahoo, AOL, and regional providers, then reports which folder each one routed to. Also does blacklist monitoring and DMARC analytics. Roughly $59 to $129 a month.
Mailgenius. A free basic placement check plus a deliverability checklist, with paid tiers for depth.
Inbox Monster. Enterprise-grade seed placement and monitoring, custom priced, for teams sending at real scale.
Run a layer 2 test weekly while a domain is warming, then monthly and before any large campaign. See inbox placement test and email deliverability test.
A warm-up tool. TrulyInbox or Mailivery for flat-rate pricing across many mailboxes, Mailreach for standalone use, or the bundled warm-up inside Instantly or Smartlead. See best email warm up tools.
DMARC monitoring. GlockApps includes it, or a dedicated DMARC platform if you need detailed reporting. It tells you who is sending as your domain and whether your alignment is holding. See what is DMARC.
Blacklist monitoring. An automated watch on your domain and IP across the major lists, so a listing pages you instead of surfacing three days later in a bad campaign.
For a cold email operation under 500,000 sends a month:
| Tool | Layer | Cost |
|---|---|---|
| Google Postmaster Tools | 1 | Free |
| Microsoft SNDS | 1 | Free |
| mail-tester | 1 | Free, paid for volume |
| MXToolbox | 1 | Free |
| GlockApps | 2 | ~$59 to $129 / month |
| Warm-up (flat-rate) | 3 | ~$29 to $99 / month |
That is roughly $90 to $230 a month for a stack that covers most of what an enterprise deliverability suite does. The free ISP tools plus GlockApps and MXToolbox alone cover about 90%.
Owning the tools is only half of it. Here is a routine that keeps a cold operation ahead of problems in about 20 minutes a week.
The point of the routine is to catch a decline at 90% inbox placement, when a week of clean sending fixes it, rather than at 65%, when you are rebuilding domains.
Solo founder, under 5,000 a month: layer 1 free tools plus a monthly mail-tester and MXToolbox check. Add a placement test only when warming a new domain. Total cost near zero.
SMB team, 5,000 to 100,000 a month: the full lean stack. Free ISP tools always on, GlockApps run weekly, one flat-rate warm-up tool for the mailbox fleet. Roughly $90 to $230 a month.
Agency, multi-client: the lean stack per client workspace, plus continuous blacklist and DMARC monitoring so a problem on one client's domains is caught automatically rather than on the weekly manual pass. See best ESP for cold email for the sending side.
The Three-Layer Stack: always run at least one layer 1 free ISP tool. Add layer 2 placement testing during warm-up and before campaigns. Add layer 3 warm-up and monitoring as volume and stakes rise. Buying above your layer wastes money.
The stack framing stops two opposite mistakes. Under-tooling, where a sender runs cold campaigns with no Postmaster Tools and no placement testing, flying blind until a domain is already burned. And over-tooling, where a small operation pays for an enterprise suite, a dedicated DMARC platform, and three testing tools, spending enterprise money on a startup's volume.
Match the layer to the situation. A founder sending 2,000 a month needs layer 1 and an occasional layer 2 test. A team sending 200,000 a month needs all three, running continuously. Nobody under half a million a month needs more than the lean stack above. The quotable version: "Buy the layer your volume is in, not the one the vendor's demo showed you."
Start with the free layer 1 tools today, add GlockApps when you begin warming domains, and add continuous monitoring when a manual weekly check stops being enough.
Deliverability tools measure and monitor. They do not change what you send to, and the metrics they track, complaint rate, bounce rate, spam placement, are all downstream of list quality.
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The email deliverability stack that works for most senders is three layers. Layer 1 is the free ISP tools, Google Postmaster Tools, Microsoft SNDS, Yahoo Sender Hub, mail-tester, and MXToolbox, and you should always have them. Layer 2 is one placement-testing tool like GlockApps, run during warm-up and before campaigns. Layer 3 is warm-up and ongoing monitoring, added as volume grows. Under 500,000 sends a month, that stack costs $90 to $230 a month and covers about 90% of what an enterprise suite does. Anything more is buying above your volume.
At minimum, the free ISP tools: Google Postmaster Tools for Gmail, Microsoft SNDS for Outlook consumer, Yahoo Sender Hub for Yahoo, mail-tester for content, and MXToolbox for blacklists. Add one placement-testing tool like GlockApps during warm-up and before campaigns, and a warm-up tool while building new domains.
For a cold email operation under 500,000 sends a month, roughly $90 to $230 a month covers a full lean stack: free ISP tools, one placement tester, and a flat-rate warm-up tool. The free ISP tools plus GlockApps and MXToolbox alone cover about 90% of what an enterprise suite provides.
No, because it only covers Gmail. You also need Microsoft SNDS for Outlook consumer mail, Yahoo Sender Hub for Yahoo, and a seed placement test for business domains that none of the ISP tools cover. Postmaster Tools is the most important single tool, but it is one of several you need.
Only if you need detailed per-source reporting on who sends as your domain. For most senders, the DMARC analytics bundled into a tool like GlockApps, plus a correctly published DMARC record, is enough. A standalone DMARC platform is a layer 3 addition for teams with complex sending or compliance needs.
A warm-up tool actively sends and replies between mailboxes to build sender reputation on a new domain. A deliverability monitoring tool measures where your mail lands and reports complaint rates, authentication, and blacklist status. Warm-up is one layer of a full deliverability stack, not a replacement for monitoring.
Mostly. The free layer 1 tools give you authoritative data from Gmail, Outlook, and Yahoo, plus content scoring and blacklist checks. The gap they leave is per-provider inbox placement testing, which needs a paid seed tool like GlockApps. Free tools plus one seed tool is a genuinely sufficient stack for most cold operations.
LSI keywords: deliverability monitoring, Google Postmaster Tools, Microsoft SNDS, Yahoo Sender Hub, GlockApps, mail-tester, MXToolbox, inbox placement testing, warm-up tool, DMARC monitoring, blacklist monitoring, spam rate
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