Receiving server connection failed
The connection to the receiving server failed before message content could be sent.
What to do now
- Check the recipient domain’s MX and server address.
- Check connectivity to the server and port.
- Check again later for a temporary server or network outage.
Representative reply
connect to mx.example.com:25: Connection timed out
Retry decision
Retry after network or server recovery; inspect the sending queue.
Why it happens
A firewall, route, port or server state may prevent connection.
Typical stage
CONNECT
- CONNECT
- EHLO
- MAIL FROM
- RCPT TO
- DATA
- DELIVERY
Stages vary by server. Confirm the actual stage from the complete reply and session logs.
What is SMTP Reply Code?Open only the explanations you need
Three-digit server reply: 4xx is transient, 5xx permanent. Read the complete text to investigate the cause.
What is Enhanced Status Code?Open only the explanations you need
An extended status such as 5.1.1. Server wording and usage may vary.
What is MX?Open only the explanations you need
DNS record identifying receiving mail servers for a domain.
What is STARTTLS / TLS?Open only the explanations you need
STARTTLS upgrades an existing connection. Implicit TLS encrypts from connection establishment.
Check directly
- Check the recipient domain’s MX and server address.
- Check connectivity to the server and port.
- Check again later for a temporary server or network outage.
If you operate or diagnose the mail server, the tools below can help with these checks.
dig MX example.com openssl s_client -starttls smtp -connect mx.example.com:25 -verify_return_error -crlf EHLO test.example MAIL FROM:<sender@example.com> RCPT TO:<user@example.com>
Cautions
Observed DSNs do not map one-to-one to causes. The stage shown is typical; confirm it with the full reply and actual logs.
Similar reply wording and observed status codes
connection timed outconnection refusedno route to hosttimed out while receiving the initial server greeting
Observed DSNs: 4.4.1, 4.4.2, 4.3.0