IBM have today released a statement explaining why some applications will be unable to connect to Domino servers from iOS9 and El Capitan devices due to Apple removing support for Elliptic curve technology (no – me either) and enhanced transport security. This doesn’t affect only IBM but it’s something you need to be aware of. There will be an interim fix for Domino 9.0.1 FP4 and also a new FP5 to resolve these issues (eta end Sept) but there will be no fix for Domino 8.5.x servers.
The full statement and explanation is here but the key summary is
Additionally, IBM is working on an Interim Fix for 9.0.1 Fix Pack 4 (and the upcoming 9.0.1 Fix Pack 5) that will implement Elliptic Curve cipher support for TLS 1.2 and TLS 1.0 that remedies this issue and implements Elliptic Curve support for the following protocols: HTTP/HTTPS, LDAP/LDAPS, SMTP, IMAP, and POP3. Currently, the ETA for the Interim Fix posting is end of September 2015.
Elliptic Curve support will not be available for Domino 8.5.x releases since the specification requires updated cryptographic libraries that are available only in Domino 9.0 and above.