Another standard feature missing in Opera's built-in mail client is support for aliases and global SMTP accounts.
Some mail providers allow you to have mutliple identities (also called aliases) for the same mail account.
Each identity has its own settings like: email address, name, signature, reply-to, whether to compose in HTML by default, what smtp account (username, password, server name, port, connection security etc.) to use, what vCard to attach, what reply style to use and what sent, drafts, archive and template folders to put messages in etc.
When you compose a new message, you can choose what identity to use. And, when you reply to a message, what identity is used for the reply by default depends on the email address the message was sent to. So, if you have email@example.com and firstname.lastname@example.org identities and someone sends you a message to email@example.com, if you reply to that message, the identity settings for firstname.lastname@example.org will be used by default instead of email@example.com's.
Further, you can then filter messages by what identity they're for and tag/mark messages by rules etc.
Having multiple identities allows you to use different email addresses all from the same account. For example, you could create a firstname.lastname@example.org identity that you use for signing up for stuff so you don't have to reveal the address for your main identity.
To do all that stuff with a mail provider that doesn't support identities requires you to sign up and create a new account for each. And, if you do, you may not be able to see messages from all the accounts in a single interface (for webmail interfaces at least) and you will probably have problems logging into all accounts at once (cookie issues). Having multiple identiities gets rid of those problems.
Mail providers that support multiple identities support this stuff in their webmail interface. But, if you want to use the mail account with a local mail client *also*, your mail client also needs to support multiple identities. Otherwise, you're going to be extremely limited and annoyed when you have to deal with messages you receive as they'll be sent to different identities and replying to the messages won't use the right email address and signature etc. But, even if a mail provider doesn't support multiple identities, you can at least use the multiple identity support in your mail client.
Gmail, Fastmail, Yahoo and Hotmail are examples of mail providers that support multiple identities. These are extremely popular services.
The problem is, Opera doesn't support multiple indentities for the same mail account at all. This means that if you use multiple identities in any of those, Opera's built-in mail client isn't much fun to use.
Now, there are things you can do to kind of hack in identities into Opera. But, they're inefficient (extra copy of a message for each identity) and replying doesn't choose the right fake identity if unless you reply to the right copy. The former you can avoid by not specifying the incoming server. But, the downside to that is you can't specify a an IMAP sent folder. There's just no good way to make it work in Opera's built-in mail client.
For an example of good multiple identity support, check out Thunderbird. On the main page in an account's settings, you'll see a "manage identities" button where you can add each new identity for the account. Check out all the settings each identity has. Each identity pretty much has all the settings a full mail account has except for the incoming server.
Now, another thing you'll notice in an identity's settings in Thunderbird is that you can choose what smtp server and account to use. This is because in Thunderbird, an smtp account is not tied to an IMAP/POP account. SMTP accounts are global (with one being the default). This provides flexibility in general and with identities. Opera doesn't support this either.