This is not Gnome, but it is GTK+ and supports multiple POP accounts.
The basic interface offered by Spruce
Copyright © 1995-2019 Togaware Pty Ltd
Support further development through the purchase of the PDF version of the book.
Brought to you by Togaware and the author of open
source software including Rattle and wajig.
Also the author of Data Mining with Rattle and Essentials
of Data Science.