421 Service not available

Gmail

• 421 4.3.0Temporary System Problem. Try again later.

  Gmail temporarily rejected the message, with no other reason. The sending server's automatic retry usually solves it.

• 421 4.4.5Server busy, try again later.

  Gmail's servers are busy; a later retry solves it.

• 421 4.7.0Connection expired, try reconnecting.

  The connection expired; the sending server has to reconnect.

• 421 4.7.0IP not in whitelist for RCPT domain, closing connection.

  The recipient's domain accepts mail only from certain IP addresses, and yours is not among them. Only the recipient's administrator can change that.

• 421 4.7.0Try again later, closing connection.

  Gmail temporarily rejected the message, with no other reason. The sending server's automatic retry usually solves it.

• 421 4.7.0The IP address sending this message does not have a PTR record, or the corresponding forward DNS entry does not point to the sending IP. To protect our users from spam, email sent from your IP address has been temporarily rate limited.

  The sending IP address has no PTR, or the PTR name does not lead back to the same address, so Gmail limits or blocks its mail. The PTR is set by whoever holds the IP address. Check with SMTP server check.

• 421 4.7.0TLS required for RCPT domain, closing connection.

  The recipient's domain requires TLS and the connection was not encrypted; the sending server must use STARTTLS. Check with Mail delivery test.

• 421 4.7.0This message is suspicious due to the very low reputation of the sending IP address. To protect our users from spam, email sent from your IP address has been temporarily rate limited.

  The sending IP address has a very low reputation at Gmail, so its mail is limited or blocked. Check blocklists, authentication and what you send; Postmaster Tools shows the reputation. Check with Blacklist check.

• 421 4.7.0This message is suspicious due to the very low reputation of the sending domain. To best protect our users from spam, the message has been blocked.

  The sending domain has a very low reputation at Gmail, and the message was blocked. Check with Blacklist check.

• 421 4.7.0This message is suspicious due to the nature of the content or the links within. To best protect our users from spam, the message has been blocked.

  The message was blocked because of its content or the links in it.

• 421 4.7.26This email has been rate limited because it is unauthenticated. Gmail requires all senders to authenticate with either SPF or DKIM. Authentication results: DKIM = did not pass SPF domain-name with ip: ip-address = did not pass.

  The message passes neither SPF nor DKIM, and Gmail requires at least one of them from every sender. Check with Mail delivery test.

• 421 4.7.27Your email has been rate limited because SPF authentication didn't pass for this message. Gmail requires all bulk email senders to authenticate their email with SPF. Authentication results: SPF domain-name with IP address: ip-address = Did not pass.

  The message does not pass SPF, which Gmail requires from bulk senders. Check with SPF generator.

• 421 4.7.28Gmail has detected an unusual rate of email. To protect our users from spam, email has been temporarily rate limited.

  Gmail noticed an unusual rate of unsolicited mail (from the IP address, the netblock, the DKIM or SPF domain, or with a given domain in links) and limits it. Slow down and make sure the recipients asked for the mail. Check with Blacklist check.

• 421 4.7.28Gmail has detected an unusual rate of email originating from your IP address ip-address. To protect our users from spam, email sent from your IP address has been temporarily rate limited.

  Gmail noticed an unusual rate of unsolicited mail (from the IP address, the netblock, the DKIM or SPF domain, or with a given domain in links) and limits it. Slow down and make sure the recipients asked for the mail. Check with Blacklist check.

• 421 4.7.28Gmail has detected an unusual rate of unsolicited email originating from your IP Netblock ip-address. To protect our users from spam, email sent from your IP Netblock has been temporarily rate limited.

  Gmail noticed an unusual rate of unsolicited mail (from the IP address, the netblock, the DKIM or SPF domain, or with a given domain in links) and limits it. Slow down and make sure the recipients asked for the mail. Check with Blacklist check.

• 421 4.7.28Gmail has detected an unusual rate of unsolicited email originating from your DKIM domain domain-name. To protect our users from spam, email sent from your domain has been temporarily rate limited.

  Gmail noticed an unusual rate of unsolicited mail (from the IP address, the netblock, the DKIM or SPF domain, or with a given domain in links) and limits it. Slow down and make sure the recipients asked for the mail. Check with Blacklist check.

• 421 4.7.28Gmail has detected an unusual rate of unsolicited email originating from your SPF domain domain-name. To protect our users from spam, email sent from your domain has been temporarily rate limited.

  Gmail noticed an unusual rate of unsolicited mail (from the IP address, the netblock, the DKIM or SPF domain, or with a given domain in links) and limits it. Slow down and make sure the recipients asked for the mail. Check with Blacklist check.

• 421 4.7.28Gmail has detected an unusual rate of unsolicited email containing one of your URL domains. To protect our users from spam, email with the URL has been temporarily rate limited.

  Gmail noticed an unusual rate of unsolicited mail (from the IP address, the netblock, the DKIM or SPF domain, or with a given domain in links) and limits it. Slow down and make sure the recipients asked for the mail. Check with Blacklist check.

• 421 4.7.28Gmail has detected an unusual amount of unsolicited email originating from your IP address. To protect our users from spam, email sent from your IP address has been temporarily blocked.

  Gmail noticed an unusual rate of unsolicited mail (from the IP address, the netblock, the DKIM or SPF domain, or with a given domain in links) and limits it. Slow down and make sure the recipients asked for the mail. Check with Blacklist check.

• 421 4.7.28Gmail has detected this sender exceeded the quota for sending messages that have the same Message-ID:. To best protect our users, the message has been temporarily rejected.

  The sender sent too many messages with the same Message-ID; each message must carry a unique identifier (RFC 5322 §3.6.4). Check with Header analyzer.

• 421 4.7.29Your email has been rate limited because you're not using a TLS connection. Gmail requires all bulk email senders to use TLS/SSL for SMTP connections.

  The message did not come over a TLS connection, which Gmail requires from bulk senders. Check with Mail delivery test.

• 421 4.7.30Your email has been rate limited because DKIM authentication didn't pass for this message. Gmail requires all email bulk senders to authenticate their email with DKIM. Authentication results: DKIM = Did not pass.

  The message does not pass DKIM, which Gmail requires from bulk senders. Check with DKIM generator.

• 421 4.7.40Your email has been rate limited because the sending domain doesn't have a DMARC record, or the DMARC record doesn't specify a DMARC policy. Gmail requires all bulk email senders to add a DMARC record to their sending domain.

  The sending domain has no DMARC record, or it has no policy (p=); Gmail requires one from bulk senders. Check with DMARC generator.

• 421 4.7.32Your email has been rate limited because the From: header (RFC5322) in this message isn't aligned with either the authenticated SPF or DKIM organizational domain.

  The From domain is aligned with neither the SPF nor the DKIM domain, as DMARC requires. Check with Mail delivery test.

• 421 5.7.32Your email was blocked because the From: header (RFC5322) in this message isn't aligned with either the authenticated SPF or DKIM organizational domain.

  The From domain is aligned with neither the SPF nor the DKIM domain, as DMARC requires. Check with Mail delivery test.

Microsoft 365 / Exchange Online

• 421 4.3.240The maximum number of concurrent server connections has exceeded a per authenticated source limit, closing transmission channel.

  The sender opened too many simultaneous authenticated connections (the limit is 250).