Oh, my mistake, i misunderstood the error log. Sorry about this.
Looks like the destination server tries to verify whether sender address (YOUR email address) exists or not.
If they're using SMTP command 'VRFY' to verify your address, then you must have "disable_vrfy_command=no" in Postfix main.cf (/etc/postfix/main.cf), iRedMail disables it by default ("disable_vrfy_command=yes"). If they use different way to verify sender address, i have no idea yet.
WARNING: If you allow to use 'VRFY' command in Postfix, others can use it to verify valid email addresses, then send you SPAM. That's why iRedMail disables it by default. Use it on your own.