The certificate it's self-signed, but i agre them in the thunderbird exceptions.
Exim log this error:
2018-02-22 14:24:20 TLS error on connection from [200.58.112.191] (SSL_accept): error:00000000:lib(0):func(0):reason(0)
2018-02-22 14:24:26 TLS error on connection from [200.58.112.191] (SSL_accept): error:00000000:lib(0):func(0):reason(0)