We've been using outlook email addresses to send outgoing emails over SMTP for a few years now without issue, however recently some of our agents noticed they seemed to be not receiving some emails seemingly randomly. After looking at the system logs we found a large number of "Mailer Error" errors. These errors seem to occur in pairs, the first error in the pair has the message "Unable to email via SMTP:smtp.office365.com:587 [email address] Failed to set sender: email address [SMTP: Failed to write to socket: unknown error (code: -1, response: )]" and then the second error in the pair says "Unable to email via php mail function:"user" mail() returned failure"
We haven't recently made any changes to the OSTicket set up and as I mentioned before these errors are seemingly random. Any advice to help resolve this would be greatly appreciated.