Will the sender retry after a 556?

No. A null MX tells senders to give up immediately, so the bounce arrives within seconds and there is no queueing.

Is 556 a block against me?

No. It applies to every sender; the domain simply states that it does not accept email.

SMTP 556: domain does not accept mail (null MX)

556Domain does not accept mailPermanent: retrying will not help

Typical wording

556 5.1.10 Recipient address has null MX

What it means

RFC 7504 registered 556 as "Domain does not accept mail". Unlike 521, which a server sends about itself, 556 is about a destination domain. It is mostly produced by a sending or relaying server that has decided, from DNS, that the recipient domain cannot receive mail, and it then records 556 in the bounce it generates.

The usual trigger is a null MX from RFC 7505: an MX record with preference 0 and a target of a single dot. RFC 7505 says a sender that finds one must not try delivery and should bounce at once with 556 and enhanced code 5.1.10 ("Recipient address has null MX"). Its sibling for senders is 550 5.7.27, used when a receiver refuses mail whose sender domain has a null MX.

Because the bounce happens immediately, there is no retry and no queue delay. That is the point: domains that never receive mail save senders days of retries.

Likely causes

The recipient domain deliberately publishes `MX 0 .` because it does not receive email.
A user typed a domain that is used only for a website or parked, and its owner published a null MX.
An intermediate relay looked up the domain and found it does not accept mail by its own rules.
A domain that used to receive mail was retired and its owner replaced the MX with a null MX.

How to fix it

Look up the recipient domain’s MX records. A single record of `0 .` confirms a null MX.
Check the address for typos; a misspelt domain that happens to publish a null MX gives exactly this bounce.
Ask the recipient for an address on a domain that receives mail.
If you own the domain and want to receive mail, remove the null MX and publish real MX records, for example `example.com. MX 10 mx1.example.com.`

Checks that apply

Your domain or sending IP (fills in the checks below)

MX Lookup Shows whether the recipient domain publishes a null MX (preference 0, target ".") or real mail hosts.
Domain Health Grades the domain’s mail setup end to end and flags a null MX when the owner did not mean to block mail.

FAQ

Will the sender retry after a 556?: No. A null MX tells senders to give up immediately, so the bounce arrives within seconds and there is no queueing.
Is 556 a block against me?: No. It applies to every sender; the domain simply states that it does not accept email.

All 191 bounce codes