I’m not sure how to read "when the email is open on the Mac", but a first theory is that your Mac is set up to use IMAP while other devices use POP, and your email provider may lock simultaneous access from both POP and IMAP as long as an IMAP connected device is open. This however is not very obvious, as there is no persistent connection in case of IMAP servers, the whole point of IMAP is to access mailboxes from multiple devices, and IMAP and POP servers probably don’t communicate to exchange this info. You probably would get an error message as well in case of other devices really being "locked out", not a "no mail" indication as I understood it. * But if so, you might be able to change the Mac setup to POP, although this is unlikely, but you may just need to recreate the account in Mail. See [https://support.apple.com/en-au/HT201855 this Apple support article] for more info - note the setting for leaving messages on the server that you may want to check intentionally in order to keep the Mac as the "master" for removal. * If not, the second theory is your Mac ''is'' set to use POP, but also to remove messages after retrieval so there is simply nothing left to download, which is more common. One option is to uncheck that option (unless you want to remove it anyway), another is to increase the POP interval, and another is to choose "When moved from Inbox", which you could consider the only valid action to have them removed from the server. The second scenario is more obvious, can be checked by letting the Mac check for mail (manually), sending an email to yourself from an iPhone right afterwards and then checking for new mail on an iPad manually while not touching the Mac meanwhile. If it works, the Mac POP settings are the culprit.