Option 3 would try to downgrade you to OpenHAB 2.4 which is not supported. I believe 2.5 is expected to be released as stable at the end of this year. The integration of ESH and redesign of the build system delayed its release.
2.5M3 is the current Testing build. i am unsure whether you are currently running a Testing build or Unstable snapshot. I would recommend the Testing version.
mail is no longer an action, but a binding. You have to uninstall the action (maybe through karaf, as it won’t emerge on Paper UI, or is it listed in /etc/openhab2/services/addons.cfg?) and install the binding instead. Please be aware that afterwards you have to change the configuration and the rules.
That now is confusing at least to newbies.
“unstable” (and “testing” and “stable”) are the Linux repository names (which in turn have another load of aliases like jessie/buster on Debian or trusty/bionic etc on Ubuntu) while when we’re talking about the openHAB packages (from the openHAB repository to only contain these and none of the rest of Linux), we’re talking about “release”, “milestone” and “snapshot”.
So naming a repo “official” in the forum here would relate rather to openHAB and not to Linux, so the “official” name would rather be “snapshot”.
And “nightly” is none of them, it’s even incorrect as new snapshot builds are not generated every night but only when explicitly triggered.
Confused and dazed now ?
Not sure just a guess: if it’s a OHv1 action (mail.cfg to exist indicates it was) you might need to install the appropriate package (openhab2-addons-legacy) and/or enable fetching legacy addons from the online repo in addons.cfg