It looks like Zimbra 8 introduced a bug that causes the "from" email address on a configured iPhone Exchange account to reset to a random alias: Bug 78903 – Multiple address for zimbraAllowFromAddress change emailaddress field for activesync
If you use multiple personas, or use a canonical address - then you may not be able to choose the "From" email address you want to display on emails coming from your iPhone. I know this is broken on my iPhone - can't speak to Android, though I imagine the bug is on that phone too as it relates to the Zimbra software, not the phone software.
The best way to get this fixed? Please vote for this bug! Tell your friends! Then hide your kids, hide your wife (from this terrible bug)!