Can't connect to remote mail

[Snowmon]

On a (new) Linux 8.2 box, whenever I try log onto a domain's remote mail, it gives an error. Its set up (as far as I can tell) identical to our other Linux box which works fine. The error is rather vague: [Error #002] Unable to access the remote service: connecting to remote session failed. Contact your system administrator for support.

Any ideas will be greatly appreciated.