zmlocalconfig -s | grep lmtp
zmlocalconfig -e postfix_lmtp_destination_concurrency_limit=20
zmprov -l gs `zmhostname`  | grep zimbraLmtpNumThreads
zmprov ms  `zmhostname`  zimbraLmtpNumThreads 40

https://wiki.zimbra.com/wiki/Performance_Tuning_Guidelines_for_Large_Deployments