[Evolution] Unable to encrypt outgoing emails

[Edward Torvalds <ltorvalds024 gmail com>]

Hi,
I am using Evolution 3.18.5.2 on Elementary OS 0.4.1.
I use GPG to encrypt my outgoing emails. But after reinstall of my
system, I am not able to encrypt outgoing email to my friends. Although
I am able to encrypt emails if I send it to myself.
The error I get is:

```
Could not create message.
Because "gpg: using subkey BB84D566 instead of primary key CEF41E5C
gpg: using PGP trust model
gpg: This key belongs to us
gpg: CC461E31: There is no assurance this key belongs to the named user
gpg: [stdin]: encryption failed: Unusable public key
", you may need to select different mail options.
```

I have imported my private and public keys in GPG. Have entered
'OpenPGP key' in settings and have trusted my own keys. I have also
made sure that my keys aren't expired.

Still can't figure out why I am getting this error. Please I need help
me out.
If I am missing any information required to solve this issue please
pardon me, and let me know. Thanks...
-- 
Edward Torvalds <ltorvalds024 gmail com>