Just had this issue. Had to bump the softlimit up to 8000000.
/service/qmail-send: up (pid 21861) 151 seconds
/service/qmail-send/log: up (pid 21862) 151 seconds
/service/qmail-smtpd: up (pid 21863) 151 seconds
/service/qmail-smtpd/log: up (pid 21864) 151 seconds