I'm trying to set up a new installation of SDP but I can't get the incoming mail settings working.

I've put the settings in according to -  https://support.office.com/en-us/article/POP-and-IMAP-email-settings-for-Outlook-8361e398-8af4-4e97-b147-6c6c4ac95353 but whenever I try to save them I get the error - FAILURE :E-mail settings saved successfully. But unable to connect with mail server - Host unknown / not reachable.

I can ping outlook.office365.com OK and get a telnet connection on port 25 from this server. I've checked username/password for the account works OK. Windows firewall is turned off.

It feels like I'm missing something obvious as it seems to be saying it's not even resolving the server name.

I'd be grateful for any help on this.