When sending mail through the squirrelmail webmail interface and using the installation defaults on Debian Wheezy and Debian Jessie, Greek characters do not appear properly in the destination inbox. This has to do with the default squirrelmail encoding which is set to iso-8859-1 .
To solve this, I had to perform the following:
View the file /usr/share/squirrelmail/functions/strings.php and search for the string $list_of_encoding=array( where you should find the word *utf-8* and remember the spelling.
Now, exit the file and run squirrelmail-configure as root in the terminal.
Type 10 and press Enter (To select Languages).
Type 2 and write utf-8 .
Logout from squirrelmail and relogin and you should now be able to send e-mails written in Greek that will be properly readable from the recipient.