IMAP NOTIFY/IDLE Heartbeat

Using IMAP NOTIFY or IDLE with FairEmail and Fastmail, it's possible to have stable idle connections with a 29 minutes heartbeat, tested with multiple devices on both my Wi-Fi and on cellular network. However, with mailbox's servers, any heartbeat longer than 15 minutes results in dropped connections. I don't know if this is because of the mail server or maybe a proxy? With both mail servers, there are no frequent notifications emitted by the server to prevent NAT resets in FairEmail's log.