Imap (The default)
Access from all popular clients using a username and password
Pros
free
universal, Imap is supported by all mail clients
tranferable, can be seen on multiple clients at the same time
Calendar built into the web client
Cons
you have to sign into each device
you'll have to resign into clients periodicaly
Can be Difficult to sign in
Apple Hosted Email
Access on Apple devices automaticaly
Pros
No need to sign in, once your device is signed into icloud your email will automaticaly appear
access on any device through icloud.com
Cons
Can take longer to load emails
forced to use the apple mail client
Can be Dfficult to move off the service
Requires an icloud storage plan
Full IT Management
automatic access everywhere
Pros
sign in once per device through a guided setup
access through any client via imap or automatic service
Calendar built into the webclient
Cons
requires a very small download onto each device
costs a small fee per seat (£5 per email account a month)