Hello! Greek characters should be fully supported by OS X. At least OS X Yosemite, which is what I am using.
Please try the following:
1/ Make sure all your software is up-to-date by going to "Apple menu > App Store" and then [updates] You don't have to upGRADE right now if you're running something earlier than Yosemite.
2/ Experiment with the following, this might help with other people reading your mails (not with you reading their mails however)
- Open Mail and navigate to the menu: "Mail > Preferences" and then the tab [NEW] The one with a pen and piece of paper.
- Whatever message formatting is set to currently, change it to the other option. For example: if your current setting is "RTF" experiment with "plain text" and visa-versa.
Now, about you receiving wrong characters in your emails, the solution will have to be found in the manner which the original authors sends them. If they are also Apple users you can ask them to do step 2. If they're Windows users, I'm afraid I'm out of my depth.