smtp_data_xfer_timeout (default: 180s)
The SMTP client time limit for sending the SMTP message content. When the connection makes no progress for more than $smtp_data_xfer_timeout seconds the Postfix SMTP client terminates the transfer.

Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks). The default time unit is s (seconds).


viz: http://www.postfix.org/postconf.5.html#smtp_connect _timeout