Frequent issues with iOS Devices
Problems Connecting to CERN Network
iOS 14 introduced a feature called 'Private Wi-Fi Address', meant to reduce tracking of your iPhone. This options changes the hardware address (also called MAC address) of your iPhone whenever it changes the network, and it is enabled by default. This can not work with the CERN network where the hadrware address of a device needs to be registered to allow connecting to the network.
Users need to disable the Private Address for the CERN network in Settings / Wi-Fi / CERN.
Problems Sending Email Messages
iOS 16 introduced a featured called 'undo sending' (see iPhone User Guide for details), which allows to cancel a sent message within a certain delay, by default 10 seconds. When a user quits the Mail.app within the defined delay the message will not be sent.