I have a CentOS VPS and I'm using Exim 4.7.2
For some days I have an issue in Thunderbird and in other client because seems STARTTLS not work in incoming email only outgoing e-mail.
How can I fix this?
It is not very clear what you're asking, but if your observations are that not all email that is received by your mail server comes in using TLS , that's completely normal.
Upgrading the SMTP connection to an encrypted one is always optional for remote smtp servers. Even at this day and age still not all mail servers are equipped with SSL / TLS support, let alone with proper certificates, so you can expect some remote email to come in unencrypted.