How could Feyris send her message in the past if she was miles away from the phone microwave?
It is logically accurate. The necessary conditions for a D-Mail to work are listed on the Wikia as:



D-Mails are sent through the Phone Microwave and can only be sent from devices connected to the Phone Microwave, such as a phone or a pager.




D-Mails can only be sent to someone with a phone (or a pager), so although they could hypothetically be sent back millions of years, they would not have any effect.




The number of the target phone is also necessary to send the D-Mail, so they cannot be sent to individuals whose numbers the sender has not obtained.



Hence, it's not a plot-hole. Basically, it functions like a normal message sent from a mobile to the Phone Microwave (name subject to change) as mentioned in the comments. The Phone Microwave (name subject to change) doesn't pick-up any random message sent in a nearby zone, which, in size, is shorter than or equal to 36 bytes but only the ones sent to it.
This also tells us that the device sending the message needs to be connected to the Phone Microwave and not the one receiving it. As mentioned by Builder_K, the Phone Microwave may be functioning as a relaying device, which relays the message to the target phone as intended by the sender.