I am running Mac 10.10.5. My virus protection app AVIRA is always deleting .EMIX files.. What are these and should I delete?

Uninstall Avira, it is no better than malware itself.

https://www.avira.com/en/support-for-business-knowledgebase-detail/kbid/1242

There are no viruses that can attack Mac.OS X. So, anti-virus programs are selling a cure for a disease that doesn't exist. And they are renowned for borking Mac systems.

A Mac can pick up malware but not if you just use common sense. You really have to actively invite some malware in for it to happen. That means downloading and knowingly giving your password for it to download. Mac provides all the protection you need with SIP (System Integrity Protection).I About System Integrity Protection on your Mac - Apple Support

If you do happen to download adware you can use MalwareBytes to get rid of it. Malwarebytes was developed by one of our own colleagues here in ASC. It gets rave reviews and is about the most proven anti-malware software for Mac.

Safari pop-ups are different and easier to deal with. In Safari 9.1 and later just close the window. For earlier versions force quit and hold the shift key while restarting Safari.

https://support.apple.com/en-us/HT203987

Finally, stay away from programs that purport to "clean your Mac". They can do damage to your system. Your Mac needs no cleaning in the first place. All the cleaning that is necessary is taken care routinely by OS X.

You should delete the virus scanner, update your Mac to OS X 10.11.5 and Mac can take care itself.

As for eMix read here: https://en.wikipedia.org/wiki/EMix

As soon as you uninstall the scam, Avira, the problem will go away.

.EMIX files are the way Macs store email.

As I said, no anti-virus should be used.

No AV software is necessary on a Mac.

Hence the reason I told you to uninstall it. AV software on a Mac offer no protection whatsoever. All it does because using an inordinate amount of computer resource is cause problems.

If you do not wish to keep your email messages then you can delete the .emix files.