If you ARE two years behind, please update! Apple doesn't backport security patches any farther than the last two OS releases. Apple users tend to also be Safari users, and Safari has a number of known security problems that will never be patched in the older versions.
But... if your Mac is "vintage" (seven years or older), Apple will not even allow you to update. Rather than buying a new Mac, you can install ElementaryOS or Ubuntu and continue using it with modern, patched software.
In that case, note that Thunderbird has to be manually downloaded in order to be compatible with GT's product-dependent choices. GT seems to think it's OK not to bother supporting Linux users. I'm sure our students with older devices can all afford a brand new Mac if they really try hard enough, right?
Supporting the most accessible systems would be a level of ethical IT that I don't think this campus has really put much thought into, honestly.