Verify EWS availability

I'm trying to connect davmail to an Exchange 2016 server. The Exchange admin says they set up EWS, but I am unable to connect either davmail or Apple Mail to the EWS endpoint. I would like to know what methods exist to test the proper availability of EWS from the general internet so I can rule out problems with davmail and Apple Mail, and have some confidence in claiming there's something wrong on the Exchange side. I know nothing about Windows administration, and they do it professionally, so I need to have something to show.

Here's what I'm seeing:

In Apple Mail, when setting up a new account, selecting "Exchange" an supplying email address and password, I get "unable to verify account name or password". I have verified the email / password combo works using Outlook Web Access on the same Exchange server. Supplying an explicit "External URL" to Apple Mail does not help.

When connecting to davmail using IMAP, I see "Authentication failed: invalid user or password" in davmail's logs.

I also tried curl, but it's not getting me anywhere either:

simon$ curl -v -u "<username>:<password>" <EWS endpoint URL>
*   Trying <IP>...
* Connected to <host> (<ip>) port 443 (#0)
* TLS 1.2 connection using TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
* Server certificate: *.domain
* Server certificate: thawte DV SSL SHA256 CA
* Server certificate: thawte Primary Root CA - G3
* Server auth using Basic with user '<username>'
> GET /EWS/Exchange.asmx HTTP/1.1
> Host: <host>
> Authorization: Basic <basic auth string>
> User-Agent: curl/7.43.0
> Accept: */*
> 
< HTTP/1.1 401 Anonymous Request Disallowed
< Date: Fri, 29 Dec 2017 23:02:40 GMT
< Server: Apache
< request-id: d6fa4e8d-bbce-4de5-a097-753cda2e37e2
< X-WSSecurity-Enabled: True
< X-WSSecurity-For: None
< X-OAuth-Enabled: True
< WWW-Authenticate: Negotiate
< WWW-Authenticate: NTLM
< X-Powered-By: ASP.NET
< X-FEServer: COMPANYMX-01
< Content-Length: 0
< Set-Cookie: ClientId=<some id>; expires=Sat, 29-Dec-2018 23:02:40 GMT; path=/; HttpOnly
< Content-Type: application/x-asmx
< 
* Connection #0 to host <host> left intact

I don't have access to a Windows machine, but I could borrow one if it helps.