Why is your server reporting "failed temporarily: network read error: end of file"?

If you have a Cisco PIX firewall also see Why am I receiving duplicate e-mails. This problem can be caused by a temporary degredation or loss of Internet connectivity. It can also be caused by problems with 'Path MTU Discovery'. Problems with IP path MTU discovery usually cause the delivery failure of all except very small e-mails. A little background is in order. With the SMTP protocol, the HELO, MAIL FROM and RCPT TO commands and responses are relatively short. Until ESMTP command pipelining was implemented, every command and every response was sent as a separate packet. The message content, however, is sent as a few datagrams, each datagram typically a kbyte large or even bigger, based on the local network MTU. When e-mail fails consistently due to a timeout, It is commonly due to problems with path MTU discovery. Our servers start by sending large packets, with the IP DON'T FRAGMENT bit set, preventing intermediate routers from fragmenting the packets that are too big for their networks. Depending on what network path a message follows, some router on the way responds with an ICMP MUST FRAGMENT message saying the packet is too big. Normally, our servers will re-send the data after chopping it up into smaller pieces. However, things break when some router closer to the receiving system is dropping such ICMP feedback messages, in a mistaken attempt to protect systems against certain attacks. In that case, the ICMP feedback message never reaches the sending machine, and the connection times out. This is the same configuration problem that causes trouble with web servers behind a misconfigured packet filter: small images/files are sent intact, large images/files time out because the server does not see the MUST FRAGMENT ICMP feedback messages. Fix: find the router that drops the ICMP MUST FRAGMENT messages, and convince the person responsible for it to fix the configuration.