Anyone who had issues with certs signed by Let's Encrypt may have noticed
issues. They had a major "oops" by not renewing their own certs which made
all other certs under them appear expired.

https://www.yahoo.com/news/internet-goes-down-millions-tech-021400230.html

I know on iPhones, email accounts that use servers who have certs signed
by Let's Encrypt have issues. I'm not an Apple person but if anyone knows
how to force an update of the cert without having to recreate an email
account I'd love to hear from you.
--- SBBSecho 3.11-Linux