I use 1.5.5.19 Aqua Mail.
It works perfect with firewall off, but when I use some apks as DroidWall, Android FireWall, AFWall+, Mobiwol, etc, it works ok thru wifi, but some times doesn't thru data network. I use two accounts and sometimes ones work ok, but the other not. So, if I diseable the firewall, it work ok full.
Others mail clientas like Gmail, Mailwise, etc work ok thru firewall on.
Please, I can send iptable chains if you like for analyze it.
Thanks