Email client IMAP IDLE settings

Currently, I’ve set my client connection keepalive to about 10 minutes, which works about 75% of the time.
Meaning that the connection does not have to be re-initiated.

What is the maximum time you server keeps a client connection open?

(Ideally, I’d like it to stay open for >20min, but that always leads to a reset on the next sync.)

Just a guess here, but does your ssl proxy have a generic 10min timeout?

You may want to increase that to 30min for :993 to be rfc compliant.