To authenticate users on our SMTP Server (exim), we are using the embedded perl interpreter to do a simple login with the passed credentials against our imap server. So in the end dovecot will do the whole authentication dance.

We used this solution for years, however while deploying newer EL7 based relay smtp servers and testing authentication, it became apparent while looking at the logs that there seems to be an issue with validating the certificate of the IMAP server:

 Using the default of SSL_verify_mode of SSL_VERIFY_NONE for client
 is deprecated! Please set SSL_verify_mode to SSL_VERIFY_PEER
 together with SSL_ca_file|SSL_ca_path for verification.
 If you really don't want to verify the certificate and keep the
 connection open to Man-In-The-Middle attacks please set
 SSL_verify_mode explicitly to SSL_VERIFY_NONE in your application.
  at /usr/share/perl5/vendor_perl/Net/IMAP/ line 135.

So it seems that newer perl-Net-IO-Socket-SSL versions finally do some proper validation and the old ones never did. Actually, the state of ssl in perl is/was quite worrisome, but things got fixed for the better in the last few years.

So all we had to do was to adapt our authentication-code (Part of the exim_imap_auth module) so that it now enforces validation of certificates. This required us to also package the latest version of perl-Net-IMAP-Simple for our distributions, as none of them have picked up the new versions so far.