Inframail vs Mailgun: cold email infrastructure vs email API in 2026

Last updated: September 2026 · Marcus Chen, Outbound sales trainer, 150k+ emails sent

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TL;DR — 5 things to know before reading

• Inframail is cold email infrastructure (dedicated Microsoft 365 inboxes); Mailgun is a developer API for programmatic email delivery — they are not the same category of tool
• Mailgun's SMTP relay is sometimes used as a cold email workaround, but the API model lacks inbox rotation, per-inbox warmup, and the deliverability controls cold email requires
• Inframail handles SPF, DKIM, and DMARC automatically at provisioning; Mailgun requires manual DNS setup documented in their own authentication guides
• Mailgun charges per email sent; Inframail charges a flat monthly fee regardless of inbox count, which is significantly more cost-effective for cold outreach volume
• For the full outbound stack, Quarvio provides verified contacts and Inframail provides the sending infrastructure

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Our take

Mailgun is one of the most technically sophisticated email API services available. Its documentation is thorough, its deliverability for transactional email is reliable, and its authentication guides are referenced across the industry as the standard for SPF, DKIM, and DMARC setup. For developers building applications that send transactional email, Mailgun is a legitimate choice.

None of that makes it the right tool for cold email infrastructure. The mistake cold email teams make is reaching for Mailgun because it supports custom domains, SMTP relay, and gives you control over your sending reputation. That control comes through an API model that was designed for programmatic transactional delivery — not for the inbox rotation, per-inbox warmup, and dedicated sending architecture that cold outreach requires. This guide explains the difference and why Inframail solves the cold email infrastructure problem that Mailgun was never designed to address.

What each product does

Inframail provisions dedicated Microsoft 365 inboxes for cold email sending. When you add a sending domain, Inframail automatically configures SPF, DKIM, and DMARC records. Inboxes connect to cold email sequencing tools like Instantly via SMTP. Flat monthly pricing covers unlimited inboxes. The product exists specifically to solve the infrastructure layer of cold email: getting properly authenticated, dedicated inboxes running at scale without manual DNS work.

Mailgun is an email API platform for developers. It provides REST API and SMTP relay interfaces for sending email from your application. You configure your domain with Mailgun's DNS records, send API requests, and emails go out through Mailgun's sending infrastructure. It is designed for developers who need programmatic email delivery: transactional notifications, account-related email, and bulk sends to opted-in lists.

Side-by-side comparison

The SMTP relay trap

The most common reason cold email senders evaluate Mailgun is its SMTP relay option. Unlike a pure API integration, SMTP relay lets you point cold email tools directly at Mailgun's servers by entering SMTP credentials — which looks similar to how you connect an Inframail inbox to Instantly. The surface similarity masks a fundamental architectural difference.

When you use Mailgun via SMTP relay, you are still sending through Mailgun's shared sending infrastructure with a single set of SMTP credentials. There is no concept of separate inboxes with independent reputations. Every email you send goes through the same SMTP endpoint, which means your sending volume accumulates on a single identity rather than being distributed across dedicated inboxes.

This matters because cold email deliverability depends on inbox rotation: spreading daily send volume across many inboxes and domains so no individual inbox or domain accumulates enough volume to trigger reputation degradation. An API or SMTP relay model cannot replicate this. The Instantly's cold email benchmarks puts the safe per-inbox limit at 30–50 emails per day for properly warmed accounts. A Mailgun SMTP relay has no per-inbox structure at all.

Authentication: the same destination, very different paths

Both Inframail and Mailgun result in emails sent with properly authenticated SPF, DKIM, and DMARC records. The difference is in how you get there.

With Inframail, authentication is automated. Provisioning an inbox creates the correct DNS records without you touching your domain registrar. Scaling to 20 new inboxes across 5 domains means 20 correctly authenticated inboxes with no manual DNS work.

With Mailgun, authentication is manual. The Mailgun SPF, DKIM, and DMARC setup guide — which Mailgun publishes and many practitioners use as a reference — walks through the steps of adding TXT records, verifying propagation, and confirming setup in the Mailgun dashboard. This process works, but for an agency or team managing many sending domains, it introduces setup time and error risk at each new domain.

The practical time difference: manual DNS setup per domain averages 30–60 minutes including propagation wait and verification. Inframail's automated setup takes under five minutes per inbox. At 10 sending domains, the difference is roughly 5 hours vs 50 minutes.

Warmup: critical for cold email, absent from Mailgun

According to Instantly's cold email benchmarks, inboxes used for cold email require 2–4 weeks of warmup before live campaigns, with full deliverability maturity taking up to 12 weeks. Warmup is the process of gradually increasing send volume and generating positive engagement signals to build the inbox's sending reputation before it touches cold contacts.

Inframail supports inbox warmup. Mailgun has no warmup mechanism designed for cold email. Mailgun was designed for transactional email, which goes to opted-in users who generally engage with it — transactional email does not require the same warmup process because it is not arriving unsolicited.

Using Mailgun for cold email without warmup means sending from an inbox that has no positive engagement history. Gmail and Microsoft's spam filters are tuned to detect this pattern. The result is elevated spam placement on early sends, which compounds into a damaged sender reputation that is slow to recover.

Pricing comparison

Mailgun charges per email sent, with costs scaling based on monthly email volume. For transactional email where each send represents a customer interaction, per-email pricing is appropriate. For cold email where you might send 10,000 emails per month spread across 20 inboxes, per-email billing compounds at a rate that Inframail's flat monthly fee does not.

Inframail's flat monthly model means your infrastructure cost is fixed regardless of sending volume or inbox count. For teams scaling from 5 to 50 inboxes, the flat model produces a decreasing per-inbox cost as you grow — the opposite of per-email pricing, which scales linearly with output.

For Mailgun documentation, the product architecture and pricing model are clearly designed for application developers, not for sales teams running cold outreach.

"The flat pricing on Inframail completely changed how I thought about scaling our outbound operation. Adding more inboxes to handle more clients was just adding inboxes — not a budget conversation every time we needed more capacity."

— verified reviewer, Inframail reviews on G2

When Mailgun is the right choice

Mailgun is well-suited for developers building applications that need programmatic email delivery: SaaS products sending user notifications, billing platforms sending invoice emails, onboarding systems sending confirmation sequences to new users. Its API is mature, its documentation is thorough, and its deliverability for opted-in transactional email is reliable.

If you are an SDR, agency, or sales team running cold email campaigns at volume, Mailgun is the wrong tool for the infrastructure layer. The API model, shared sending infrastructure, and per-email pricing were designed for a different buyer with different requirements.

The full outbound stack

Inframail sits at the infrastructure layer of a complete cold outbound stack. It provisions the Microsoft 365 inboxes that Instantly sends through. Quarvio provides verified B2B contact data for those campaigns. Aimfox handles the LinkedIn channel alongside email sequences.

For more on infrastructure setup, see the cold email deliverability guide, the Inframail review, and the SPF, DKIM, and DMARC setup guide.

Our actual stack

Frequently asked questions

Can I use Mailgun's SMTP relay for cold email?

Technically yes, but using Mailgun's SMTP relay for cold email creates the same structural problems as using the API directly: no inbox rotation, no per-inbox warmup, shared sending infrastructure, and per-email pricing that does not suit cold outreach economics. Inframail is built specifically for the cold email use case and solves these problems at the infrastructure level.

What is the main difference between Inframail and Mailgun?

Inframail provides dedicated Microsoft 365 inboxes with automated DNS authentication, flat monthly pricing, and SMTP compatibility with cold email sequencing tools. Mailgun provides an email API and SMTP relay for programmatic email delivery, requires manual DNS setup, and charges per email sent. They are different categories of product designed for different buyers.

Does Inframail automate DNS setup like Mailgun's guide describes?

Yes. Inframail automates the entire DNS authentication process that Mailgun's documentation describes as a manual workflow. SPF, DKIM, and DMARC records are configured automatically when you provision an inbox in Inframail. You do not need to follow manual DNS setup steps.

What is Mailgun best used for?

Mailgun is an excellent API platform for developers who need to send transactional email programmatically: account confirmations, password resets, billing notifications, and onboarding sequences to opted-in users. Its documentation is strong and its deliverability for transactional email is reliable. It is not designed for cold outreach.

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Cold email infrastructure starts with the right inboxes.

Quarvio provides pre-verified B2B contacts as a one-time purchase — no subscription, 12-month credit validity, unused credits returned. Pair verified contacts with Inframail for managed Microsoft 365 inboxes and Instantly for sequences to complete the outbound stack.

Start your order on Quarvio →