SMTP code 421 indicates the server is temporarily unable to process the request and is closing the connection. Unlike 220 (ready), 421 tells the client that the server exists but cannot handle mail right now. This can appear at any point in the SMTP conversation — as the initial greeting instead of 220, or mid-session if the server encounters a problem. The client should treat this as a temporary failure and retry according to its queue retry schedule, typically starting at 15-30 minutes and increasing exponentially.
The SMTP server has reached its maximum number of concurrent connections or is running low on resources (CPU, memory, disk I/O). It returns 421 to shed load, expecting clients to retry later when the load subsides.
Many mail servers implement greylisting, which temporarily rejects mail from unknown sender/recipient/IP combinations with a 421. Legitimate sending servers will retry and succeed on the second attempt, while spam bots typically do not retry.
During planned maintenance or daemon restarts, the SMTP service may briefly return 421 to all incoming connections. This is usually resolved within minutes once the server finishes restarting.
The mail server may be unable to perform necessary DNS lookups (reverse DNS verification of the connecting IP, SPF checks) due to DNS resolver issues, causing it to temporarily reject connections with 421.
421 is explicitly a temporary failure. Most mail transfer agents (Postfix, Exim, Sendmail) will automatically queue and retry. If you are sending manually, wait 15-30 minutes and try again.
Verify that the mail server's SMTP ports are reachable from your network. A 421 at the greeting usually means the server is running but overloaded, while a connection refused means it is down entirely.
telnet mail.example.com 25Scan Ports
If the primary MX is returning 421, check whether backup MX servers are configured. Mail should fall through to lower-priority MX records if the primary is unavailable.
dig MX example.com +shortCheck DNS Records
Some servers issue 421 if the connecting IP does not have valid reverse DNS (PTR record). Check that your sending server's IP has a PTR record that resolves back to the IP.
dig -x YOUR_SENDING_IP +short
If you recently changed MX records, the old server may still be receiving mail while DNS propagates. Verify that DNS changes have propagated globally.
Check PropagationThe SMTP server is ready to begin a mail transaction and has announced its greeting banner.
The recipient's mailbox is temporarily unavailable. The server will not process the command right now but the sender should retry.
The server encountered an internal error while processing the command. The message should be retried later.