Nothing. The only thing you can do is use an unencrypted connection, but then you're just trading one set of security issues for an even worse set.
Besides, email scanners aren't really necessary. Follow a few simple rules, and the odds of getting hit by any nasty payload in an email is virtually zero.
1: Do NOT use Outlook (which you have down, but others may read this)
1a: It is a favorite target for ne'er do wells. The reasons why can be debated by those with no lives, we only care that it is, and there's no reason to paint a giant target on your back when there are literally dozens of equally good (if not better) email clients out there
2: Do NOT open ANY email attachments you aren't expecting
2a: If someone you know sent you an email attachment, BEFORE opening it, send them a message requesting confirmation that they intended to send you an email with an attachment
3: Do NOT click on ANY links in emails
3a: Copy and paste the URL into the browser, give it a quick once over looking for things like multiple http:// entries, making sure that the link you pasted into your browser is the same as was in the email, THEN you can visit it if nothing seems awry
3b: It wouldn't hurt to follow the suggestions of 2a, and ask the sender what the link is
4: NEVER trust email scanners to do an effective job scanning messages as they come in
4a: ALWAYS save attachments, then manually scan them before opening
Most of the things AV programs do, beyond just scanning your system for viruses, is a lot of smoke and mirrors. They don't really add anything to the overall security of your computer, but the AV companies have to do something to justify people buying new versions. The number of threats that come in via IM programs, for example, is probably so miniscule for all intents and purposes it doesn't exist, but there's an IM scanner in almost every AV program now. You have to wonder if any of the messages being scanned are being collected and transmitted back to the AV company to resell to marketing companies. Just because a company claims not to be doing anything of the sort doesn't always mean they're telling the truth, or that some PR rep is even aware that it is actually going on. You just follow a few basic rules, like those I outlined above, and you're going to be far more effective at evading and avoiding malicious payloads in emails than any AV scanner will ever be.
I use Thunderbird for email.
A message has started popping up again and again, saying "Email scanner is not able to scan messages because connection is encrypted."
What do I have to do so that the email scanner will be able to scan messages?