I also had the exact same error:Failed to connect to **** And for whatever reason, when I put ssl:// in front of the SMTP server name it worked as well.I just also migrated my server one host to another as well.I spent around 2.5 hrs searching for this solution, so here are some keywords to help people find this solution:Keywords: "osticket server migration", "osticket error Invalid response code received from server", "osticket SMTP error"Thank you mfelber for your answer.