IIRC from what my senior explained to me few years ago, this has to do with the inward and outward aspect of the Japanese business society and the Japanese language itself. The Japanese honorific is usually used from inward to the outward (e.g. to stranger, customer, partner, etc.)

I think the easiest way to think might be that you won't refer to your sister/brother with honorifics when talking about them with someone.

When you (an outsider) email a company, everyone in the company who replied to the email are considered belonging to the same group, so they should not use honorifics to refer to themselves regardless whether they are CEO or just random employee (even if you call them with honorifics in internal email.)

There's also another small rule regarding this, such as you can say something like "Uchida, the CEO" (CEOの内田) but NOT "CEO Uchida" (内田CEO) because the title in the latter is being used as a suffix, and considered a honorific (equivalent to -san). It's a weird rule.

Talking about your own “team” honorably (-san) is not humble. To be fair Japanese people have trouble with it all the time too; There are tons of results for on Google for phrase + polite or phrase + business