I'm aware of this.
Aqua supports Google's own "Cloud Print" on 4.4 and up, also Cloud Print by Paolo Fernandes, also any other app that you can "share into"...
And since it works just fine with any other app capable of receiving text, I have to believe the issue is in this printing app, not Aqua.
Sanity check:
I don't own an Epson printer, but did install the app, and tried to share/print from AquaMail. Got a preview of two blank pages (so they are both blank, but the Epson app "thinks" there is something there, worth two pages).
Then I selected some text in Gmail and "shared" it into Epson iPrint, got same exact two blank pages.
If I "share" an attachment (image) from Aqua into Epson iPrint, then I get the expected preview. So seems that iPrint is geared towards working with files, not scraps of text.
Feedback-AndroidAppsPrn@exc.epson.co.jp