This varies widely by niche. My experience is that a solid majority of West Coast tech companies / startups use Gmail or other non-MS hosted solutions. Outlook or MS365 are a good indicator that the codebase may be older than some of the people writing it.
Silicon Valley in particular uses Google Workspace at a much higher rate than the rest of the world. If you count every one- or two-person startup as a company, Google probably does have a solid majority. If you count mailboxes, Microsoft still easily wins.
Note that MX records are misleading here. They have no false positives, but are full of false negatives --- daisy-chaining MTAs is common, and since Microsoft owns the mailbox, it's invariably last in the chain. So the MX record will show something like Proofpoint (pphosted) or Mimecast or an internal company host, when really it's Microsoft in the end.
