Most likely it is blocked by your firewall.
The solution is to change configuration of your SMTP server on AS400. I did it using iSeries Navigator.
In iSeries Navigator go to iSeries/Network/Servers node (you should click into plus sign(+) at the coprresponding nodes shown in the left pane of the iSeries Navigator window.
Double click on TCP/IP node under Servers. Right side of the window will list all TCP/IP servers. Find SMTP among them and dDouble-click it (or right click and select Properties).
A new window with SMPT properties will open. Look at the General Tab at Mail router group of fields.
You most likely have field “Forward outgoing mail to router through firewall” checked.
You have choice either uncheck this field (probably is not the best solution) or add list of e-mail domains which will bypass firewall to the list below (“Do not forward the following domains through the firewall”). For example, if you are sending e-mail to firstname.lastname@example.org add aol.com to the list. Then click OK to save changes.
I think you should restart SMTP server for this change to take effect (right-click on SMTP and chose Stop, then right-click again and chose Start)