Agency sales conversations are almost entirely about results. Case studies, meeting counts, a screenshot of a calendar. Almost nobody asks about the infrastructure that produces them, which is strange, because deliverability is the variable that decides whether any of it works.
It is also the easiest thing to check. Most of what follows you can do from public DNS records and one test email, before you spend anything.
The point is not to catch anyone out. It is that the agencies who can answer these questions comfortably are a materially different tier of supplier from the ones who cannot, and the difference is invisible in a case study.
Nine checks
1. Ask for a sending domain and look up its records
Ask which domains they would send from on your behalf, or for an example from an existing client. Then check the DNS yourself.
You want SPF present and not exceeding the ten-lookup limit, DKIM with a 2048-bit key, and DMARC published. A missing DMARC record in 2026 is a genuine problem given Gmail and Yahoo’s bulk sender requirements. Run it through a domain health checker if you would rather not read the records by hand.
An agency that cannot produce a domain to check, on grounds of confidentiality, can produce one of their own. They do outbound for themselves.
2. Ask how many mailboxes per domain
The answer should be a small number, ideally three, with five as a hard ceiling. If they run ten or fifteen mailboxes on a single domain, they are optimising domain cost against your sender reputation, and one bad campaign takes out everything on that domain.
3. Ask about warmup duration
Three to four weeks per mailbox is correct. “We warm for a few days” or “our provider handles it” means either they do not know or they compress it to hit a start date. Both are the same problem.
Follow up with: what happens if warmup metrics look bad at day ten? A real answer describes pausing and diagnosing. A vague one describes proceeding.
4. Ask whether the domains will be registered in your name
The most consequential question in the whole conversation, and the subject of its own post. The answer you want is that domains sit in your registrar account with delegated access for them.
5. Send yourself a test
Ask them to send a sample sequence email to an address you control. Then look at the headers.
Check that SPF, DKIM and DMARC all pass. Check whether the tracking domain is a custom one on their sending domain or a shared tracker used across every client. A shared tracking domain means your links carry the reputation of every other client’s campaigns.
Check where it lands, too. Primary, Promotions or Spam tells you something immediately.
6. Ask what their list sources are
The answer should describe verified data with a real-time verification step before send. If the answer includes buying lists or scraping without verification, expect bounce rates that damage the domains you are paying for.
Ask what bounce rate they consider acceptable. Anything above 3% as a working number is a red flag; good operators aim well below 2%.
7. Ask how they monitor placement
Sending volume and open rates are not placement monitoring. You want to hear about seed testing or inbox placement tests run on a schedule, with per-provider results.
Then ask what they do when placement drops on one provider but not others. A specific answer, something about authentication, content, or that provider’s reputation thresholds, indicates someone who has actually debugged this. A general answer about “optimising the copy” does not.
8. Ask about blocklist handling
Who monitors, on what cadence, and who handles delisting. If their infrastructure sits on a shared pool they do not control, delisting is somebody else’s job on somebody else’s timeline, and that timeline is your pipeline.
9. Ask what happens when a client’s campaign goes badly
This is the question that separates operators from resellers. If several clients share sending infrastructure, one client’s aggressive list damages everybody on that infrastructure, including you.
The answer you want describes separation: one IP set or domain estate per client, monitored independently. The answer you do not want is that it has never happened.
Reading the answers
You are not grading a test. You are listening for whether these are familiar questions.
An agency that runs its own infrastructure answers quickly, with numbers, and often volunteers detail you did not ask for. An agency that resells somebody else’s platform answers in brand names: they use a provider, the provider handles it, the provider is very good.
The second category can still deliver perfectly acceptable results. But when something breaks, they are opening a support ticket on your behalf, and you should price that in.
The four that matter most
If you only ask four:
- Will the domains be registered in our name?
- How many mailboxes per domain, and how long is warmup?
- How do you monitor inbox placement, and what do you do when it drops?
- Is our sending infrastructure separated from other clients?
Those four cover ownership, setup quality, ongoing operations and blast radius, which is most of what determines whether a programme works past month three.
The short version
Deliverability decides whether an agency’s copy ever gets read, and it is the part of their service that never comes up in a sales conversation. Most of it is checkable before you sign: look up the DNS on a sending domain, send yourself a test and read the headers, and ask four direct questions about domains, warmup, placement monitoring and client separation.
The agencies who enjoy these questions are the ones worth hiring. That is most of the signal right there.
Next: SPF, DKIM and DMARC explained, or who owns your sending domains.
Frequently asked questions
How do I know if a cold email agency is any good?
Check the technical setup rather than the case studies. Look up SPF, DKIM and DMARC on a domain they send from, ask how many mailboxes they run per domain and how long they warm for, and ask how they monitor inbox placement.
How many mailboxes should an agency run per domain?
Three is the sensible working number and five should be the ceiling. More than that concentrates too much sending on one domain’s reputation.
Can I check an agency’s email deliverability myself?
Yes. Ask for a sending domain and check its DNS records, then have them send you a sample email and inspect the headers for SPF, DKIM and DMARC passes and for a custom rather than shared tracking domain.
What bounce rate should a cold email agency maintain?
Below 2% as a working target. Anything they describe as acceptable above 3% suggests list sourcing that will damage the domains you are paying for.