PGP public key
If you go to Menu > Tools > Settings > Signing and Encryption > Certificates and Keys, if you do not already have a key you can create one.
Once you have a key pair, you can export your public key by clicking on the settings cog next to your key and choosing Export > Export PGP Public Key.
In Menu > Tools > Settings > Signing and Encryption > Account policies, you can also choose to have the public key attached to all outgoing emails.