They change for the same reason website fonts change - if the system doesn't have it installed (or active) it's better to display something rather than nothing. For the most part it is fine, until you start using extended characters that don't translate between different platforms & font formats.
There is no font that everybody has since there are a billion and one variations of OS's, user choices & installed software. Licensing means some OS's will never have the fonts available on Macs & PC's (I don't have Avenir for example).
Stick with Helvetica on Mac & PC's will might show Arial that looks close enough (or choose to use Arial), unless they have prefs set for Times.
Verdana & Georgia are other 'old school web' fonts that may be available on older systems (and have reasonable fall-backs). If they don't like Times & their mail reader forces it upon them it's hardly your fault.
Some other old web safe fonts…
http://www.angelfire.com/al4/rcollins/style/fonts.html
You should ask whoever you are emailing if they actually care, I'd suggest people mostly want to just read the content, unless you are creating newsletters that need to look pretty, then you may be able to use html messages (and annoy the 'plain text' only users). Selecting fonts & styling (bold, underline, italics, inline links with titles) also changes messages from plain text to rich text or html which can annoy people who use old email clients.
If you want an example of how Apple may go with this feature look at iOS, 5 years on there are still no font choice in Mail, Mac OS also dropped font preferences that changed the entire UI, no way to set a default mail font…
Re:co
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