Hello, since the IIex does not control the radio
you need two serial connections (and therefore COM ports) from your computer - one connecting to the modem
to receive data and one connecting to your radio to control the radio. If you use USB rather than DB9 ports
(9 pin male serial ports
used on older computers) on your computer, the software
included with your USB-DB9 converter creates virtual or software
COM ports, usually numbered 3 or higher and whose number can change if you move the converter to a different USB port. It sounds as if Airmail trying to control the radio using different virtual COM port than is created by the USB-DB9 converter.
Go into your device manager (assuming Windows 7/8) and see what COM ports are configured for which USB-DB9 adapter, and make sure Airmail is set up to use the correct port for the converter that is used to connect to the radio.