Point your mail here.

A black hole for unit tests. The reply is fast, and the message is discarded. No signup, no inbox, no forwarding.

Mail it.

Invent any address. The tag after + decides the reply.

Reply

ada@fakeemaildomain.com: Accepted, then dropped.
ada+550@fakeemaildomain.com: 550 on the command that carries it. Any code from 100 to 599.
jon@550.fakeemaildomain.com: Same refusal, as a subdomain. Needs a wildcard MX.
ada+5.1.1@fakeemaildomain.com: 550 5.1.1 User unknown.

When

ada+mail-550@fakeemaildomain.com: Rejects MAIL FROM.
ada+rcpt-550@fakeemaildomain.com: Rejects the recipient.
ada+data-552@fakeemaildomain.com: Accepts the envelope. After the dot, 552. The first data code sticks.

Trouble

ada+delay-2s@fakeemaildomain.com: Waits, then replies.
ada+drop@fakeemaildomain.com: Closes the connection.
ada+hang@fakeemaildomain.com: Never answers.
ada+greylist@fakeemaildomain.com: 450 once, then 250 when that sender retries.

Try an address.

Type an address and read the dialogue. EHLO also advertises PIPELINING, 8BITMIME, SIZE, SMTPUTF8, and STARTTLS.

Whitelabel it.

Point your own domain here and send to addresses on that name. Alias a mail host, aim the MX record at it, and plus addresses such as user+550@yourdomain.com work immediately. Add a wildcard so user@550.yourdomain.com reaches this exchanger too.

CNAME

Alias your own mail host to ours, then aim the MX record at that alias. If this address ever changes, your mail follows the name. The wildcard covers every subdomain.

mail.example.com.: CNAMEfakeemaildomain.com.
example.com.: MX 10mail.example.com.
*.example.com.: MX 10mail.example.com.

Buy me a coffee.

The exchanger is free. PayPal and Cash App.