Created attachment 235249 [details]
test2.c

When playing with EWS I realized I'm not reasked for a password when the first is wrong, thus users cannot re-enter it (in evolution-ews they can reenter it, but the "authenticate" callback is not called on a session which failed before). My test program is close it, though I cannot mimic exactly that behaviour. Nonetheless, the attached test2.c shows that, with NTLM, the POST is done with only one "authenticate" emission. Only change the "exchange.example.com" with your EWS enabled exchange server host name.

Mine current output of the test is:
--------------------------------------------------------
queue_message_idle_cb: [0]: to be send

(process:27884): libsoup-WARNING **: NTLM single-sign-on by using /usr/bin/ntlm_auth failed
   authenticate_cb: retrying:0
   > queue_cb: [0]: result:401
queue_message_idle_cb: [1]: to be send

(process:27884): libsoup-WARNING **: NTLM single-sign-on by using /usr/bin/ntlm_auth failed
   authenticate_cb: retrying:0
   > queue_cb: [1]: result:401
queue_message_idle_cb: [2]: to be send

(process:27884): libsoup-WARNING **: NTLM single-sign-on by using /usr/bin/ntlm_auth failed
   authenticate_cb: retrying:0
   > queue_cb: [2]: result:401

--------------------------------------------------------

while I expect to see there:
   authenticate_cb: retrying:1
too. Testing with evolution-source-registry with configured evolution-ews account may help too, as it does call the authenticate_cb only once, on the first message, and the other messages are processed without that signal being emitted.