I don’t entirely disagree, but while deltas can be small emails are usually pretty small themselves. It might be a single 4k write to each mailbox either way. And remember that appending to a file requires the OS to do a read plus a write, while adding a file to a directory requires a write plus an update to the directory itself (another read+write at minimum). Both require allocating sectors, which means updating the freelist(s), Of course there are filesystems like ZFS that can batch up all of those writes so that they become linear writes instead of random writes, but that’s not available if you‘re running Exchange.

I have no doubt that email servers could be better optimized, but only because I have that optimistic belief about all software as a general rule. (The probability that we have already found the most optimal way to write any non–trivial program you happen to examine is pretty low.) On the other hand, I doubt that Exchange has made _no_ progress at all over the decades.

The fact that their email server fell over from the load probably has more to do with poor choice of hardware than any software flaw. Their Exchange server apparently fell over while sending email to “thousands” of people, while the Bedlam DL3 incident at Microsoft involved a mailing list with 13k people on it. In 1997. Maybe that order of magnitude difference means that the Senate needs to buy an NVME disk. (Of course that ignores the fact that asking thousands of people to send you their location via email is pure stupidity; this is the Senate we’re talking about.)