At Fit Small Business, we use G Suite and have been very happy with it. It’s only $5/month per user, and you get to use a familiar inbox with video conferencing, a shared calendar system, and access to Google Docs/Sheets.
Of course, we don’t have quite as many employees. For 150 email accounts, its probably worth looking into a dedicated email server. I don’t personally have experience setting this up, but it might be possible to run at a significantly lower price than the $750/mo required for Google. Of course, you wouldn’t get access to the same user-friendly inbox & tools then.