EnterraHost
Mail on a phone is the easy half. Every tier reads over IMAP, and the built-in mail app on any phone will add the account with your address and password. The part that catches people is the calendar.
Mail works everywhere, the same way
Add the account as an IMAP account, use your full email address as the username, and let the phone work out the server details from the domain. If it refuses to guess, the incoming server is IMAP over SSL and the outgoing server is SMTP with STARTTLS, and the username is the same for both.
One thing to get right at this point. If the account uses two-factor authentication, generate an application password before you start. A phone cannot answer a prompt for a code during setup, and the failure looks like a wrong password rather than a missing second factor.
The calendar is where the tiers differ
On Lite, the mailbox carries mail only. Adding it to a phone gives you mail, and your calendar stays where it was.
On Pro, the mailbox also carries calendar and contacts, and they sync using the standard protocols that phones already understand. That means the calendar you see in the webmail is the same calendar on the phone, and an appointment added on one shows up on the other without you copying anything.
This is the single clearest difference between the two tiers, and it is worth checking before you buy if you have assumed a phone will sync your calendar either way. The business email page sets out what each tier includes.
More than one mailbox on one device
On Pro you can add every mailbox on the domain to the same app and reach them from one place, which is what most small teams want rather than signing in and out. On Lite each mailbox is separate.
If it stops working later
A mailbox that suddenly asks for a password again is usually an application password that was revoked, or a password that was changed on the webmail without updating the phone. Regenerating the application password and re-entering it on the device fixes both, and resetting a mailbox password covers the other case.
Setting the mailbox up in the webmail first, before touching the phone, is worth the extra minute. It proves the address and the password work, so a failure on the phone is unambiguously the phone.