I bought Zimbra Network Edition, around £2k with 25 mailboxes.

Professional enough to resell accounts and works nicely on phones.

It supports high availability, mx relays and takes scheduled backups to directory of choosing.

I've been satisfied with support and it's a rock solid product that's finally starting to mature after many company take overs.

I'm having PTSD flashbacks since I had a startup based on it when VMWare owned it where we M&A'ed small mail hosting businesses and migrated them to our ZNE instances. Our license costs totaled maybe $15k USD for a dozen instances and we had access to VMware staff on Sand Hill.