Postfix delay?

[Orestes leal <orestesleal13022@cha.jovenclub.cu>]

Hi people and good morning!

The little problem it's the following, I have a small server for the lan, i've setup postfix
and ran ok but every time that I or my users send an email the window of the MUA (outlook, sylphedd)client that saids 'conecting to the smtp server at IP' or something like that and show the progress, this window *always* stay for about 20 seconds or so, instead of send the email quickly without waiting, one detail it's that i have a relay host directive with my relay server at my ISP, I have been trying with the disable_dns_lookups = yes in main.cf but I don't know how fix this problem, as always some help be gratefull received

Best,
Orestes.