We have now fixed this issue by contacting our provider. Apparently SMTP ports are blocked by default in Linode (https://www.linode.com/blog/linode/a-new-policy-to-help-fight-spam) and we'll just have to request them to open the ports for us to enable the outbound connection.
Re: SMTP : Connection failed. Error #2: stream_socket_client(): unable to connect
by Ey Marieb -
Number of replies: 0