Choosing an SMTP Provider for Cold Email, and Why Most Will Not Take You

An SMTP provider accepts your message over SMTP and transmits it to the receiving server on your behalf, using its own IP addresses, its own reputation and its own queueing. You are renting a path out of the building. It settles one question — whether a message can leave and be accepted — and no other.

The disclosure first. We build Keaz Signals, which sells buying signals and contacts for ecommerce outreach. We sell nothing in this category. No relay, no mailboxes, no domains, no warmup, and none planned. We have no product to rank first in a list of SMTP providers, which is also why this article is not that list.

Most comparisons in this category rank relays on price per thousand messages, uptime and how many free messages you get. Those are real numbers and they answer a real question — for transactional mail. For cold outbound they answer the wrong question first, because the relay that is cheapest per thousand is often the one whose terms forbid the list you were planning to send to. That is the layer under the decision, and we took the layer itself apart in What Email Infrastructure Does, and What It Cannot Decide.

Most transactional SMTP providers will not accept cold email because their own terms of service forbid the list, not because of a hidden policy or a filter you can tune around. The prohibition is written down, it is public, and it is usually a clause you can read in under a minute before you sign up.

Postmark's terms of service are a plain example. Under its permitted-use section the document states:

All email lists contained and/or used with respect to the Service must be permission-based subscriptions. Use of a list that has been purchased or rented from a third party is prohibited.

That is the vendor's own published document, retrieved 5 October 2026, and it is quoted here rather than paraphrased for a reason: the clause is specific about what it bans and we do not want to make it broader than it is. It governs the list, not the subject matter. We are not going to tell you what proportion of transactional relays carry a clause like it — we did not survey the category, and a percentage invented for this article would travel further than the caveat attached to it.

What matters practically is that the clause is checkable. Before you compare price or throughput, open the provider's terms and acceptable-use policy and find out whether your list is permitted. If it is not, nothing further about that provider is relevant to you.

Three products get shopped for under the same search and they are not substitutes. A transactional relay carries application mail. An email service provider carries marketing mail to a subscribed list. A cold outreach platform carries sequences to people who have not subscribed, from many small mailboxes rather than one large IP.

• Transactional relay. Receipts, password resets, shipping notices — one recipient, triggered by something that recipient did.
• Email service provider. Newsletters to a list that opted in. Built for throughput, segmentation and unsubscribe handling.
• Cold outreach platform. Sequences from many mailboxes on secondary domains, paced per mailbox, with reply detection.

The structural difference is who owns the reputation. On a shared relay your sending moves a reputation other customers depend on, which is why the terms read as they do. On a mailbox-per-domain setup the reputation you build or burn is yours. That is a difference in where consequences land, not a claim about anyone's compliance.

Whatever path you choose, Google's sender guidelines set the floor. Every sender to personal Gmail accounts must set up SPF or DKIM, give sending domains or IPs valid forward and reverse DNS records, transmit over TLS, format messages to RFC 5322, and keep the spam rate reported in Postmaster Tools below 0.3% (retrieved 5 October 2026).

The same document is more exacting elsewhere than the headline number suggests: it asks senders to keep spam rates below 0.10% and to avoid ever reaching 0.30% or higher. Above 5,000 messages a day it adds SPF and DKIM together, a DMARC record whose policy may be set to none, alignment between the From: domain and the authenticated domain, and one-click unsubscribe on marketing and subscribed messages.

A good provider gives you most of this correctly configured on day one. What it cannot give you is the spam rate, which recipients produce. What the three records prove is worth reading on its own: authentication shows a message came from where it claims. It says nothing about whether it was welcome.

Price per thousand is the wrong first test because the thing you are buying is not throughput, it is permission to keep sending. A relay at a fraction of a cent per message that suspends your account in week three has cost you the campaign, the domains warmed for it and the time, none of which appear on either price list.

The reason suspension is likely rather than theoretical is in Google's own guidance to providers. Its advice to email service providers is that when clients use your service to send email, you are responsible for their sending practices, and that you should immediately remove any client who uses your service to send spam (retrieved 5 October 2026). A provider enforcing its terms is doing what the receiving side asks of it.

We are not going to put a comparison table of named providers and their unit prices in this article. What each one meters is not the same thing — messages, contacts, mailboxes, dedicated IPs — and a table that lined them up as if they were would be dishonest arithmetic dressed as a recommendation. The checkable things are not prices anyway: does the AUP permit your list, is the IP dedicated or shared, and what does the provider do when a complaint arrives.

Decide in three steps, in this order, because each one removes options from the next. First the list: who you are writing to and why they would expect it. Second the path: transactional relay, ESP, or mailboxes on secondary domains. Only third the provider, chosen inside the path the first two decisions left you.

1. The list. Cold outbound to businesses is a different list from a subscriber base, and the terms you can sign follow from which one you have.
2. The path. For cold outreach this is almost always mailboxes on secondary domains through a sending platform, not a relay.
3. The provider. Read the AUP for the list clause, check whether the IP is shared, and find the suspension policy before the invoice.

Once it is running, the questions change from choosing to operating — watching reputation, pacing volume, retiring a burned domain. How to check domain reputation covers the first of those, including why a new domain shows you nothing at all for a while.

We sit above this layer and sell none of it. No relay, no SMTP credentials, no mailboxes, no domains, no warmup pool, and none planned. If you have no sending path at all, that is your first purchase and nothing on this site solves it. Buy it, then come back.

What we sell is which stores are worth contacting this week, with contacts attached. Sending runs through Instantly, in your own workspace, on your own domains, with campaign status and replies syncing back. That boundary is deliberate: the reputation stays yours, and so does the responsibility for it. As of September 2026 we watch 4.1 million Shopify and WooCommerce storefronts across Europe and North America, re-scraped weekly, with roughly 41,200 carrying a fresh signal in a seven-day window. Those are our own figures and they move.

Where we lose, plainly. We do not export — leads move into campaigns and stay there, which for some teams is a dealbreaker and should be. No LinkedIn signals, no LinkedIn sending, none planned. Shopify and WooCommerce only, Europe and North America only. Registry, firmographic and hiring triggers are marked planned on our own site, which means they do not exist. We are GDPR-conscious by design and will not claim more than that; responsibility for what you send stays with you.

We are not going to tell you how many providers in this category ban cold lists. The one clause quoted above is the one we read; we did not survey the market, and a share expressed as a percentage would be a guess wearing a decimal point.

We are not going to tell you how much better a signal-led list performs on a given sending path. It is the claim this article sits closest to and we cannot make it: it would need the same team, the same offer, the same domains, the same week, one variable changed. We did not run that experiment and we have not seen anyone else publish one.

And we make no deliverability or reply-rate promises of any kind. The refusal is the point rather than an aside. The loudest numbers in this category are the ones nobody can support, and a provider comparison built on them would rank whoever wrote the best landing page. If you want the evaluative comparison we can honestly make, it is on outreach agents compared, and the stores themselves arrive through our ecommerce leads database.

• Email sender guidelines, Gmail Help — authentication, DNS, TLS, spam-rate thresholds and the guidance to email service providers on removing clients who send spam, retrieved 5 October 2026
• Postmark terms of service — permission-based subscription requirement and the prohibition on purchased or rented lists, retrieved 5 October 2026
• Keaz Signals store coverage and weekly signal counts — our own figures, as of September 2026
• Keaz Signals product scope, including what is not shipped — our own buying signal catalogue, retrieved 5 October 2026