How do I know if a company uses Mimecast?

Look up the domain's MX records. A first MX host ending in mimecast.com means mail goes through Mimecast. The email provider lookup and MX lookup tools show it for any domain.

Are domains behind Mimecast catch-all?

Some are and some aren't; it depends on how each company configured Mimecast. A verifier can tell by offering a random address that can't exist: if the domain accepts it, the domain is catch-all and single addresses there can't be confirmed.

What does 550 Invalid Recipient mean from Mimecast?

Mimecast documents it as the reply when its recipient validation (known recipients, LDAP or SMTP call forward) found no valid internal user. The address doesn't exist at that company, so treat it as invalid.

Security gateway · Mimecast

Verifying addresses at domains behind Mimecast

Can you verify addresses at domains behind Mimecast?

Depends on the domain

It depends on the company. Domains behind Mimecast are probed like any other.

Mimecast answers for the company's own mail rules, so the checker treats every domain on its own: it offers a random address first, and if that is accepted the domain is catch-all and results come back risky ("Accepts all addresses"). Otherwise your address gets a per-mailbox answer. It always checks the syntax, the domain's MX records, disposable domains and role addresses such as info@ first.

Reasons you'll see in results:Mailbox existsMailbox not foundAccepts all addresses

Email address

Five checks a minute, no account needed. Nothing is sent to the address — we ask its mail server whether the mailbox exists, then disconnect.

Companies with MX records at regional mimecast.com hosts route inbound mail through Mimecast. Each domain chooses one of six recipient validation modes; most check a directory and reject unknown users with 550 Invalid Recipient, but one accepts every address, and those domains come back catch-all.

Live DNS snapshot: mimecast.com

Checked 21 October 2026, 07:22 UTC

mimecast.com is an MX host suffix, not a mail domain, so this is mimecast.com: a domain whose mail runs through Mimecast. Read from DNS at the time shown and refreshed at most once a day.

MX2 hosts

10service-alpha-inbound-b.mimecast.comMimecast
10service-alpha-inbound-a.mimecast.comMimecast

First MX host belongs to Mimecast.

SPFPublished

v=spf1 redirect=aojpw1q8._spf._d.mim.ec

DMARCPublished

p=reject: meets the requirement and enforces it.

v=DMARC1; p=reject; rua=mailto:a3045a7b8fd4920@rep.dmarcanalyzer.com,mailto:74e31d82-60cd-440c-ae8a-75b8a5c2ad94@rep.uk.mimecastdmarcanalyzer.com; ruf=mailto:a3045a7b8fd4920@for.dmarcanalyzer.com; pct=100; fo=1;

Look up any other domain with the MX lookup or the email provider lookup.

Mimecast at a glance

Operated by	MimecastSource: Mimecast: gateway configuration
MX hosts	Regional pairs such as `us-smtp-inbound-1.mimecast.com` and `us-smtp-inbound-2.mimecast.com`, published at the same prioritySource: Mimecast: gateway configuration
Address rule	Six recipient validation modes per domain, from directory users only and known recipients to SMTP call forward, plus one that accepts all inbound mailSource: Mimecast: recipient validation
Address rule	When validation finds no valid internal user, Mimecast replies `550 Invalid Recipient`Source: Mimecast: SMTP error codes

Why Mimecast-protected domains can be catch-all or per-mailbox

Domains with MX records at regional hosts such as us-smtp-inbound-1.mimecast.com route mail through Mimecast. Each domain picks one of six recipient validation modes, from directory users only to accepting everything.

With directory, known-recipient or call-forward validation, unknown addresses get `550 Invalid Recipient` and the check gives a per-mailbox answer. With "Accept all inbounds", every address looks valid, and the honest result is catch-all.

What our checker does behind Mimecast

SimpleVerifier doesn't skip or special-case Mimecast. For each company domain it offers a random address that can't exist, then your address, and disconnects without sending. If the random address is accepted, the domain is catch-all and its results come back risky with "Accepts all addresses". If it is rejected, your address gets a real answer for that mailbox.

Mimecast documents one accept-all mode among six, so expect a mix: domains that validate recipients get per-mailbox answers, and domains set to accept every inbound come back catch-all.

Cleaning a list full of Mimecast domains

Split the list by domain, not by platform. Two companies on Mimecast can behave in opposite ways, so a catch-all result at one says nothing about the next. The bulk catch-all checker finds which domains in a list accept everything before you spend time on single addresses.

For catch-all domains, keep addresses with recent replies or sign-ups and send the rest in a small first batch, suppressing anything that hard-bounces. For per-mailbox domains, trust the verdict: invalid addresses will bounce, and valid ones were accepted by the company's own directory. Accept-all vs unknown results covers the difference.

Common questions

Can you verify addresses at domains behind Mimecast?: It depends on the company. Domains behind Mimecast are probed like any other. Mimecast answers for the company's own mail rules, so the checker treats every domain on its own: it offers a random address first, and if that is accepted the domain is catch-all and results come back risky ("Accepts all addresses"). Otherwise your address gets a per-mailbox answer. It always checks the syntax, the domain's MX records, disposable domains and role addresses such as info@ first.
How do I know if a company uses Mimecast?: Look up the domain's MX records. A first MX host ending in mimecast.com means mail goes through Mimecast. The email provider lookup and MX lookup tools show it for any domain.
Are domains behind Mimecast catch-all?: Some are and some aren't; it depends on how each company configured Mimecast. A verifier can tell by offering a random address that can't exist: if the domain accepts it, the domain is catch-all and single addresses there can't be confirmed.
What does 550 Invalid Recipient mean from Mimecast?: Mimecast documents it as the reply when its recipient validation (known recipients, LDAP or SMTP call forward) found no valid internal user. The address doesn't exist at that company, so treat it as invalid.

Catch-all checkerTest whether a domain accepts any address before you trust a valid.

Bulk catch-all checkerFind the catch-all domains in a whole list at once.

Email provider lookupName the mailbox provider or gateway behind a domain.

Bulk provider lookupGroup a list's domains by Google, Microsoft, gateway and the rest.

Got a list with Mimecast addresses?

Upload the CSV and see how it splits into valid, risky and invalid before you pay. Then clean it with a one-time pack: $9 for 2,500 emails, credits that never expire.

See your list before you pay $9 for 2,500 emails