No luck so far. I have 500+ logs related to this in the past 5 days now. What's weird is that it still seems to only happen sometimes while other tickets / emails do not result in any issues. Is osTicket actually using the user's IP address while sending the email or does it use the IP address of the server it's installed on?PHP 5.6.23SMTP settings are confirmed to be correct and the SPF records seem to be fine since I can send mail using the php mail() function just fine using my other scripts.Unable to email via SMTP.myhost.com Failed to set sender: system@myhost.com [SMTP: Invalid response code received from server (code: -1, responseAnd these seem to be a new type..Unable to email via SMTP.myhost.com Failed to set sender: support@myhost.com Unable to email via SMTP.myhost.com Failed to set sender: careers@myhost.com [SMTP: Invalid response code received from server (code: -1, response: NԀӻ