In this particular situation I am suggesting to run postfix into verbose mode and see whether any error message is occurred. You will need to edit /etc/postfix/master.cf file.
You have to find out ‘smtp inet n - - - - smtpd -o smtpd_proxy_filter=127.0.0.1:10025’ and make following changes
After that run below mentioned command.
smtp inet n - - - - smtpd -v -o smtpd_proxy_filter=127.0.0.1:10025
Alos make changes into /etc/postfix/master.cf.
It would disable internal proxy, greylisting and antivirus. See whether it makes any difference into the situation.
smtp inet n - - - - smtpd -v -o smtpd_proxy_filter=127.0.0.1:10026