Try adding this to your main.cf

smtp_destination_concurrency_limit = 1
smtp_destination_rate_delay = 3s

@shufilk

If you happen to be sending e-mail through an application, most have built-in functionality that allows you to break up bulk e-mail and only send every X seconds or Y minutes, or should. I know way back in 2005, forum software (vBulletin) was using something similar, so modern applications should have something similar to prevent all e-mail being sent at the same time.

You can modify Postfix configuration as well, as noted by @hansen , though I would also look to your application to see if there are configurable options there as well.

Thanks for support ,
am using default Post-fix configuration only extra i have added below value ,
but i don’t know below settings will meet my requirement .

smtpdestinationconcurrencylimit = 1
smtpdestinationratedelay = 3s

Regards,
Shufil