A domain transfer moves the registration between registrars. Almost everything else about the domain is supposed to carry across unchanged, and "supposed to" is why a short checklist afterwards is worth the ten minutes.
The failures here are quiet. A transfer completes, the site keeps working, and something has broken that nobody notices for weeks.
Check the nameservers first
The single most important check, and the one that decides whether anything else matters.
Nameservers usually carry across, and occasionally the new registrar applies its own defaults instead. If that happens, your domain is now answered by the registrar's DNS, which does not have your records, and the site stops.
dig NS example.com +short
Confirm those are the nameservers you expect. If they are not, set them back immediately. There is more in updating your nameservers.
Check the records themselves
If the domain used the registrar's DNS rather than your host's, the records live at the registrar, and they do not transfer with the registration. The new registrar starts with an empty zone.
The website usually still works, because the A record was recreated or because the nameservers point at your host. What is missed is everything nobody looks at.
dig MX example.com +short dig TXT example.com +short
MX records missing means mail stops arriving, and the sender gets a bounce you never see. TXT records missing means SPF and DKIM are gone, so your mail starts being treated as spam: gradually, which is why it takes weeks to notice. There is more on rebuilding them in understanding SPF, DKIM and DMARC.
Take a copy of your records before transferring. It turns this from reconstruction into pasting.
Re-enable the domain lock
The lock has to be off for a transfer, and the new registrar does not always re-enable it.
An unlocked domain can be transferred away by anyone who obtains the authorisation code. Turn it back on the day the transfer completes: it takes one click and it is the cheapest protection a domain has. What domain locking is picks it up from there.
Check the expiry date
A transfer normally adds a year to the registration. Confirm it did.
Then check auto-renewal, which is off by default at many registrars and does not carry across from the old one. A domain with auto-renewal silently disabled expires quietly, and expiry is the worst thing that can happen to a working domain. Enabling auto-renewal goes into turning it on.
Confirm a payment method is attached too. Auto-renewal with an expired card fails the same way as no auto-renewal.
Check the contact details
The registrant contact is who legally holds the domain. Verify it is you or your company, spelled correctly.
The email address on it matters more than it appears: renewal notices, expiry warnings and transfer authorisations all go there. An address on the domain itself is a poor choice, if the domain breaks, so does the warning about it.
Also check whether privacy protection survived. It is often a separate product that does not transfer, and a domain that was private for years can quietly become public with your address in it. For that, see WHOIS privacy protection.
DNSSEC, if you had it
This one breaks the domain completely rather than partially.
If DNSSEC was enabled, the DS record at the registry must match the keys of whoever now signs your zone. A transfer that moves the registration without carrying the DS record correctly leaves resolvers expecting signatures they cannot verify, and the domain becomes unresolvable for anyone using a validating resolver.
The safe sequence is to disable DNSSEC before transferring and re-enable it afterwards. If you did not, check it now. There is more on verifying the chain in understanding DNSSEC.
Certificates are usually fine
A certificate is tied to the domain name rather than to the registrar, so it stays valid through a transfer.
Renewal is the thing to watch: automatic issuance needs to verify the domain, and it can only do that if DNS still resolves to your server. If the nameservers changed during the transfer, renewal fails weeks later with no obvious connection to what you did. Managing AutoSSL explains the failure being visible only in a log.
The sixty-day lock
After a transfer, most domains cannot be transferred again for sixty days. That is a registry rule and nobody can waive it.
It matters if you were planning to consolidate domains in stages, or if you transferred to the wrong place. Plan the destination once rather than moving twice.
Changing the registrant's details can also start a sixty-day lock at some registrars, which surprises people updating a company name.
A ten-minute checklist
Nameservers still yours. MX and TXT records present. Lock back on. Expiry extended and auto-renewal on with a valid card. Contact email one you read, off this domain. Privacy still active. DNSSEC consistent or disabled.
Then send yourself an email to an address on the domain, and load the site over HTTPS. Those two checks together catch most of what the list above is protecting against. There is more on the process itself in transferring your domain.
The same checks apply after an account-to-account move, which does not always update the registered owner. How to Move a Domain Between Registrar Accounts walks through that case.
The same checks apply after a domain is moved by the registry rather than by you. What Happens If Your Registrar Goes Out of Business goes over when that happens.
Set a reminder for the first renewal
A transferred domain renews with the new registrar on a date that may differ from what you are used to, and the notice goes to whatever address the transfer left in place. Check that address is one you read, and put the renewal date in a calendar rather than relying on the notice arriving. The first renewal after a transfer is the one most often missed, because everybody assumes the arrangement carried over intact and nobody checks which parts did.