We see many times (386) :
STARTTLS: write error=syscall error (-1)
Some (all?) of these mails do not arrive at the receipient.
The errno=104 means that the other endpoint (outlook) unexpectedly closed the connection. There's not much information about this error, and none is recent. It seems to be related to outlook.com only, and possibly to certificates (https://warthog9.dreamwidth.org/25503.html) or to greylist (https://groups.google.com/g/comp.mail.sendmail/c/GgDKMQqdDrg).
What's sure is that it's a Microsoft bug. To avoid triggering the Microsoft error, it's possible to try and play with certificates or with the greylist and see if there is an improvement.