e-mail delivery issues

Recently we have received and confirmed issues with delivering e-mail to certain domains, so far:

adelaide.edu.au
applemusic.com
arizona.edu
benwhale.com
biologists.com
communications.ssrn.com
editorialmanager.com
email.arizona.edu
flinders.edu.au
herts.ac.uk
hireright.com
hpeprint.com
inhousevtm.com
insideapple.apple.com
ioppublishing.org
mail.coinbase.com
math.arizona.edu
medicalschemes.com
mq.edu.au
newcastle.edu.au
nonzeroratio.com
skao.int
sussex.ac.uk
sydney.edu.au
telegram.com
uct.ac.za
unimelb.edu.au
uon.edu.au
uow.edu.au
uws.edu.au
vistaprint.com
webofscience.com
westernsydney.edu.au
woodpartners.com
woolcock.org.au

The mechanism by which this fails is getting clearer, but we are still figuring out how to deal with this. What most of these domains have in common is that they are hosted by the same service provider (mimecast.com). It appears to be something with the negotiation failing before the mail can be sent, which was no problem before as mails took another route after delivery failure.

Update 2024-08-13

We have established that upgrading our sendmail from 8.15 to 8.18 fixes the problem. Somehow sendmail 8.15 has a problem with STARTTLS that is fixed in later versions.

Update 2024-08-14

All smtp servers are running the new binary.