Its no surprise to anyone that gmail has held the record for the most spam on a fairly consistent basis over the last few years, but in recent months, outlook.com has now overtaken google for the top spot.
Our Anti-spam and antivirus product allows us to track spam from automated detection, and from our customers reports, allowing us to monitor trends in spam source over time. Generally when a domain, usually a company with a poorly secured mail server, is detected originating spam we're quick to black list this domain until the volume of mail hitting the gateway reduces to a more reasonable level, but with companies like gmail.com and outlook.com we can't just block those or the Help-Desk would implode with tickets. In these cases we can only blacklist certain senders, and that's a race that we can never win since spammers can register up accounts quickly and will switch as soon as they detect their email's are bouncing. So we offer customers a choice to block both *@outlook.com and *@gmail.com (and their local variants) or not, and where not, we will still try and block senders as quickly as we can.
As you would expect neither Microsoft or Google take any notice of spam reports, so our actions in blocking email have to stand in isolation. We do offer our block lists of course to anyone who wants to clone them, and we report spam to various public reputation and blacklist sites.
As an option, our customers who want gmail and outlook as senders, are able to use serverside rules to route outlook. and gmail. into a spammy folder to prevent the majority of spam arriving in the inbox, but its a short measure really.
So, whilst the rest of the planet is working hard to reduce spam, Google and Microsoft between them account for 43% of it, and this won't change as long as 'free-mail' is a thing. Just for the record, GEN used to have a free-mail product back in the 90's called GENMAIL, but we retired that a decade ago for this very reason; abuse, but of course google and microsoft aren't doing it for free. Google digests email content received and sent by its gmail users to help target advertising to them, and its likely that Microsoft does the same.
If you are a business, and can't stretch to the few pounds a month it costs to have a domain and email then seriously consider how many potential customers will never receive your replies because of blacklisting or spam detection, and make the smart choice.