Also, no matter how many sendmail servers you have you can't get around the fact that egress still takes bandwitdth.
I just got a large, image-filled email from a vendor, and it came out to 20KB (including headers). Let's assume Microsoft's announcement emails are that huge, and that Microsoft sends out 100,000,000 of them. Let's further assume that Outlook is smart enough to batch recipients to the same domain with a conservative 10-to-1 reduction in number of unique messages sent (probably closer to 500-1, given the number of Gmail users you can collapse). That math works out to about 1000 gigabit ethernet seconds, or about about 1 second of AWS's estimated bandwidth-time, or about 3 seconds of Azure's estimated bandwidth-time, or about a second of traffic at a major porn site. And that's with hugely conservative worst-case estimates for all the numbers involved.
Egress doesn't take nearly the bandwidth you might think it does.