Most companies with less that 100k employees have outsourced their email. Even those above that head count have strong incentive to outsource. Email is actually kind of difficult to do right at scale. The people that know how have their own little community. Much of the software that used to be used to implement email at massive scale is no longer publicly available, or was always private (Yahoo & Gmail). MS Exchange used to have trouble breaking 1M inboxes, requiring hundreds of hosts. Sun's Comms suite was once the telco standard, hosting 100's of thousands of inboxes per server, has now been subsumed into Oracle Cloud as their messaging service.
There's been a distinct lack of innovation too. Most of the major innovators are now dead or well past 70. Core RFC's have languished for years and even decades. John Postel has been gone since the 90's, Ned Freed, who created much of the MIME extensions passed in the early 2020's. The other big names, Eric Allman, Wietse Venema, William Yeager, are all in their 70's & 80's. The protocols are considered mature, work well with the occasional hairball, and powerful interests have built walled gardens, and are content to print money with them.
Honestly, the whole Internet email ecosystem is ripe for someone to come along and disrupt things.
T
