Special characters or symbols can be inserted via copy & paste in the subject line. Here you can find some symbols:

Dingbat

Miscellaneous Symbols

Please observe two recommendations:

• Make sure you just use special characters in a way the subject is still understandable, if the symbol can not be represented.
• Special characters / symbols will not be displayed in every email client. Please test whether the display of the symbol works properly in the different clients, for example with the Inbox Preview of promio.mail.