This may be more of a generational problem than anything else. This is the first I've heard of using two spaces after a colon, and I've only rarely seen two spaces after a period (usually from non-native English speakers). I can understand a business wanting to get rid of such a practice in professional emails as it does give the impression of being a non-native English speaker.
Looking into this further, I found this information on the topic: Chicago Manual of Style: One or Two Spaces?
A. Published work these days rarely features two spaces after a period. In the era when type was set by hand, it was common to use extra space (sometimes quite a bit of it) after periods, a practice that continued into the first half of the twentieth century. And many people were taught to use that extra space in typing class. But introducing two spaces after a sentence-ending period—and only after those periods—causes problems. Absolute consistency is easy to monitor when double spaces are never allowed, but less easy when some spaces after periods are double and others single (such as those at the ends of abbreviations and initialisms in running text). Since there is no proof that an extra space actually improves readability—as your comment suggests, it’s probably just a matter of familiarity—CMOS follows the industry standard of one space after a period.
So, looks like the practice of using two spaces is a carry over from older technology usage that has largely faded out.
Granted, punctuation rules and spacing in general started as publisher specific conventions, and there is still some level of regional variation (which is why my first thought upon seeing double spacing would be the person is not from my country).
On the other hand, there are still many English "rules" taught in some schools that are not real English rules, but rather Latin rules imposed by Latin scholars trying to teach English at some time.