Where does email sent to *@example.com go? [closed]

Solution 1:

If you attempt to send an email to *@example.com

1. Your SMTP will check the domain exists.
2. Your SMTP server will lookup for a MX record at example.com.
3. There is none: Your SMTP will fall back on the A record. The IP is 174.137.125.92 (as of today)
4. The IANA has registered the domain, but has not set up a SMTP server listening on port 25 on 174.137.125.92.
5. Then the behaviour depends on your SMTP. Most servers will send you a warning, and try again later. Eventually (usually in 3 days), the SMTP will discard the message and send you a notification of failure.

Bottom line: It depends on your own configuration. But if IANA set up a server today, they might be able to receive messages you tried to send 3 days ago.

Solution 2:

If there's no MX record, mail servers will attempt deliver to the A record.

example.com's servers don't listen on port 25, so the mail server won't establish a TCP connection and won't even begin delivery.

Solution 3:

example.com has no MX record, so your SMTP server on the sending domain should bounce the message if configured as most SMTP servers are.

EDIT: for clarity to those who find this answer in the future, here is an explanation of what an MX record is: (from http://en.wikipedia.org/wiki/Mx_record retrieved November 21, 2011)

A mail exchanger record (MX record) is a type of resource record in the Domain Name System that specifies a mail server responsible for accepting email messages on behalf of a recipient's domain and a preference value used to prioritize mail delivery if multiple mail servers are available. The set of MX records of a domain name specifies how email should be routed with the Simple Mail Transfer Protocol.

So, basically, example.com, example.net, and example.org have no server appointed to handle incoming mail, and therefore any mail sent to them should be returned to the sender as "undeliverable" (may vary based on SMTP server configuration, but returning to sender as "undeliverable" is a very common behaviour for this situation).

EDIT 2: Someone brought up the RFC 5321 defined behaviour of falling back to using the A record in the case of a missing MX record. I searched this RFC ( https://www.rfc-editor.org/rfc/rfc5321 ) and found no such thing, but it's possible that some MTAs (Mail Transfer Agent, such as exim, postfix, sendmail, and Microsoft Exchange Server, among others) may try to deliver mail via SMTP to the address defined in the A record. For posterity, here is what happens when you attempt to establish an SMTP connection to the defined A record address for example.com (192.0.43.10 at time of writing):

$ telnet 192.0.43.10 25
Trying 192.0.43.10...
telnet: Unable to connect to remote host: Connection timed out

EDIT 3: see replies below for clarifications on relevant RFCs and fallback behaviour.