Another SMPT connect error

I have an intranet WP site. We are using MS Exchange, and our CA is a our ow microsoft server. Unfortunately I cannot give access.

Email servers are working fine… I can connect to webmail and such in my browser.

I cannot get the WP Mail SMTP to connect though. My intranet site does not (yet) use SSL. My MSExchange servers do.

This is the relevant part of the debug log:
Connection failed. Error #2: stream_socket_enable_crypto(): SSL operation failed with code 1. OpenSSL Error messages:error:1416F086:SSL routines:tls_process_server_certificate:certificate verify failed [E:\www\*******\wp-includes\class-smtp.php line 374]

Any ideas what might be going on?

Full log (masked out parts for privacy)

Versions:
WordPress: 5.3.2
WordPress MS: No
PHP: 7.3.12
WP Mail SMTP: 1.9.0

Params:
Mailer: smtp
Constants: No
ErrorInfo: SMTP Error: Could not connect to SMTP host.
Host: <<mail server name>>.<<domain name>>.ca
Port: 587
SMTPSecure: tls
SMTPAutoTLS: 

E:\www\*******\wp-content\plugins\wp-mail-smtp\src\Debug.php:111:boolean true

SMTPAuth: 
E:\www\*******\wp-content\plugins\wp-mail-smtp\src\Debug.php:111:boolean true

Server:
OpenSSL: OpenSSL 1.1.1d  10 Sep 2019
Apache.mod_security: No
SMTP Debug:
2020-04-01 15:25:22 Connection: opening to <<mail server name>>.<<domain name>>.ca:587, timeout=300, options=array ()
2020-04-01 15:25:22 Connection: opened
2020-04-01 15:25:22 SERVER -> CLIENT: 220 <<mail server name>>.<<domain name>>.ca Microsoft ESMTP MAIL Service ready at Wed, 1 Apr 2020 08:25:22 -0700
2020-04-01 15:25:22 CLIENT -> SERVER: EHLO <<sitename>>.<<domain name>>.ca
2020-04-01 15:25:22 SERVER -> CLIENT: 250-<<mail server name>>.<<domain name>>.ca Hello [***.***.***.***]250-SIZE 37748736250-PIPELINING250-DSN250-ENHANCEDSTATUSCODES250-STARTTLS250-AUTH GSSAPI NTLM250-8BITMIME250-BINARYMIME250 CHUNKING
2020-04-01 15:25:22 CLIENT -> SERVER: STARTTLS
2020-04-01 15:25:22 SERVER -> CLIENT: 220 2.0.0 SMTP server ready
2020-04-01 15:25:34 Connection failed. Error #2: stream_socket_enable_crypto(): SSL operation failed with code 1. OpenSSL Error messages:error:1416F086:SSL routines:tls_process_server_certificate:certificate verify failed [E:\www\*******\wp-includes\class-smtp.php line 374]
SMTP Error: Could not connect to SMTP host.
2020-04-01 15:25:34 CLIENT -> SERVER: QUIT
2020-04-01 15:25:34 SERVER -> CLIENT: 
2020-04-01 15:25:34 SMTP ERROR: QUIT command failed: 
2020-04-01 15:25:34 Connection: closed
SMTP Error: Could not connect to SMTP host.