If i remember correctly from my old ISPCONFIG2 server - this is SASL's job?
ISPConfig 2 and 3 use a totally different mail setup. In the setup you used to install ISPCOnfig 3, smtp authentication is handled by dovecot and not sasl, so sasl must and may not be installed.
If you have a problem with smtp authentication, then please post the exact error messages from the mail log file. A common error for users that used ispconfig 2 before is that they use a wrong username for email. In ISPConfig 3, the email username is the fulle email address and not just the part in front of the @ sign.