When I delete email on my iPad or iPhone it will still shows up on my other devices, including iMac, when I open Mail. Is there a way to delete one time instead of on each device separately?
That depends on the type of service your email account uses. If you have email accounts setup as IMAP on all your device then they should all synch. Exchange accounts should also synch across your devices. If your email accounts are set as POP then that option has a host of settings to get your devices to sync. General POP email accounts do not synch between devices.
My recommendation is to work with your email service provider and learn, from them, how to setup the account as IMAP if it isn't.
Axel F.