Another standard feature missing in Opera's built-in mail client is support for connecting to an LDAP server to lookup Contacts.
This is useful for having contact info of all the employees/customers on a server that all the employees can access. The contact list can be shown as a group in the mail client's address book and can be used to autocomplete addresses when you type. And, when the contact list is updated, everybody gets the updates.
Sylpheed, Claws Mail and Thunderbird support LDAP for example. Opera doesn't support this at all, which is another downside to using Opera for business.