Every email platform offers both. The shared domain works instantly and needs no DNS. The dedicated domain needs SPF, DKIM and DMARC records and a warm-up period. The marketing copy rarely tells you which one you should be on, because the honest answer depends on a number.

The number is your monthly volume

Reputation is built from consistent volume. A receiving mail server forms an opinion about a sending domain by watching it over time. If you send 200 messages a month from your own domain, there is not enough signal for anyone to form a positive opinion, and you get treated as an unknown sender indefinitely.

Rough guidance:

  • Under a few thousand a month: shared domain. You will get better placement riding an established reputation than you will building your own from nothing.
  • Consistent five figures a month: dedicated. There is enough volume to establish and hold a reputation, and you stop being exposed to other senders.
  • Anything you brand: dedicated, regardless of volume. If the from address needs to say your company name, that is a product requirement, not a deliverability one.

What you are actually buying with a shared domain

You are renting someone else's reputation, and rented reputation is pooled. The other tenants affect your placement. A platform that runs a shared domain well is one that polices who may send under it, because the alternative is that one abusive tenant degrades delivery for everyone.

Questions worth asking a provider before you rely on their shared domain:

  • Can a free account send under it, and how much?
  • Are addresses on it screened for brand impersonation?
  • Is there a per-account ramp, or can a new account send at full volume on day one?
  • Is reputation pooled per provider account, or segmented?

A provider that cannot answer these has not thought about the failure mode, and the failure mode lands on you as unexplained spam placement.

What you take on with a dedicated domain

Your reputation becomes yours. That is the whole point and also the whole risk. A bad month is your bad month, and nobody else's volume dilutes it.

You also take on the setup. SPF authorises the sending service. DKIM signs the message so it cannot be tampered with in transit. DMARC tells receivers what to do when the first two disagree, and it is the one people skip. A domain with SPF and DKIM but no DMARC record is a domain anyone can spoof with no consequence.

The subdomain most people get wrong

Send bulk mail from a subdomain, not your root domain. If mail.example.com gets a reputation problem, example.com is largely insulated, and your invoices and password resets keep arriving. Sending marketing and transactional traffic from the same domain means one bad campaign can stop your password resets.

Separate them further if the volumes are different in kind: transactional on one subdomain, marketing on another. Receivers form separate opinions, which is exactly what you want.

Switching later

Moving from shared to dedicated is not a flag flip, it is a warm-up. Plan several weeks. Do not move all traffic at once, and do not start with your least engaged recipients. The next post in this series covers the ramp.