There are a few problems which can arise with the use of encryption technology, so it is necessary to recognise these shortcomings before using the tools.
Firstly, it doesn't protect your emails from showing who is sending and receiving encrypted emails. Although those conducting surveillance are unable to read the content of the message, they could be aware of the people involved.
Secondly, the subject line of an email is not encrypted and therefore is exposed.
Thirdly, once an email has been received, decrypted and read it can easily be sent on by simply pressing reply or forward, in which case the message may not be encrypted. You need to trust who you email!
Finally, encryption technology cannot easily and safely be used on a smartphone. Copying private keys to a smartphone could be a security risk, so many people avoid mixing phones and email encryption tools.