I can't click the lock button - so I can't encrypt mails?
An unclickable lock button in a light-grey color can have two reasons.
1. The mail-address of the sender-account used in Mail.app is not connected to a private key.
Have you already created your private key? If so, please make sure that the mail-address in the key perfectly matches the mail-address used in Mail.app > Settings > Accounts. Otherwise if you don't have a private key, you won't be able to encrypt mails. See our Tutorial "Generate a key" on how to do that.
2. No public key is found for the recipients mail-address.
- if you have a key matching the recipients mail address (in GPG Keychain)
- if the key isn't disabled or has expired
- search the key servers (again via GPG Keychain) if there is a public key for the mail-address in questions
If you have a public key from your friend but it doesn't have an email address matching the one you want to use in mail, and if you know this is the correct public key, see the following tutorial to connect the email address with the existing public key. Visit the KB-article "How to add an e-mail address to an existing public key".