The app lets you check your farm's mailbox remotely by clicking on the Email app in the mobile phone interface.
Email subject titles are supplied by the game and unfortunately are codes rather than readable subjects.
Once a mail is opened, it is removed from the email list and, like vanilla mail, cannot be read again.
Technical
Requires SMAPI, uses Harmony.
Compatible with Mod Updater for automatic updates.
Code is at https://github.com/aedenthorn/StardewValleyMods.
If you want to complain or ask for help or help me test my mods, you can visit my Discord server.
A list of all my mods for Stardew Valley is available at https://www.nexusmods.com/stardewvalley/articles/895.