It appears that sendmail is sending DATA command before RCPT, breaking ability to send mail. RFC821 - "There are three steps to SMTP mail transactions. The transaction is started with a MAIL command which gives the sender identification. A series of one or more RCPT commands follows giving the receiver information. Then a DATA command gives the mail data. And finally, the end of mail data indicator confirms the transaction." Device: Buffalo WZR-HP-AG300H 3.10.71-rc1 #10284 Thu Mar 5 01:34:58 CET 2015 mips GNU/Linux DD-WRT V24-sp2 std Command: sendmail -S <server>:<port> -d <domain> -f <from> -au<username> -ap<password> -v <rcpt><<EOF Subject: test Body EOF Stdout: sendmail: send:'NOOP' sendmail: recv:'220 REDACTED ESMTP' REDACTED 220 REDACTED ESMTP sendmail: recv:'250 OK' REDACTED 250 OK sendmail: send:'EHLO REDACTED' sendmail: recv:'250-REDACTED hello [REDACTED], REDACTED' REDACTED sendmail: recv:'250-HELP' REDACTED sendmail: recv:'250-AUTH LOGIN PLAIN' REDACTED sendmail: recv:'250-SIZE 31457280' REDACTED sendmail: recv:'250-PIPELINING' REDACTED sendmail: recv:'250-8BITMIME' REDACTED sendmail: recv:'250 OK' REDACTED 250 OK sendmail: send:'AUTH LOGIN' sendmail: recv:'334 REDACTED' REDACTED 334 REDACTED sendmail: send:'' sendmail: recv:'334 REDACTED' REDACTED 334 REDACTED sendmail: send:'' sendmail: recv:'235 ... authentication succeeded :: REDACTED' REDACTED 235 ... authentication succeeded :: REDACTED sendmail: send:'MAIL FROM:<REDACTED>' sendmail: recv:'250 <REDACTED> sender ok' REDACTED 250 <REDACTED> sender ok sendmail: send:'DATA' sendmail: recv:'503 need RCPT before DATA' REDACTED 503 need RCPT before DATA sendmail: send:'QUIT' sendmail: recv:'221 REDACTED closing connection' REDACTED 221 REDACTED closing connection