This problem is similar to Email SSL Errors after Update to 2.4.0.beta4. I suggest you update your SMTP to use TLSv1.2 and a DH key size of at least 2048 bits.
As a workaround, you should be able to add sed commands to the run section at the end of app.yml to remove the following two settings from the /etc/ssl/openssl.cnf file.
MinProtocol = TLSv1.2
CipherString = DEFAULT@SECLEVEL=2