The "reply to" address you are referring to is the Destination Email Address you provide. The SMTP field prior to that is for specifying the email server itself. The guidlines for configuring the "Modify Source Email Name" are located on page 14 of this manual - http://bit.ly/1rkdpn0
Now normally you can place anything in that field and it will be "allowed", if it isn't allowing you to change it I would look at weather you have the proper permissions to change it. That field also would need to be the internal domain name (for example VRTX@esx.local, which wouldn't be .com. You can try via the ip address would look like VRTX@10.10.10.10.
Let me know what you find.
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