curl -v --ssl-reqd  --url 'smtp://pro1.mail.ovh.net:587' --user 'username@gmail.com:password' --mail-from 'username@gmail.com' --mail-rcpt 'john@example.com' --upload-file mail.txt --ntlm

What would be the netxms setup then?

It doesn't work. I tried with two different accounts to be sure the password was not involved. I had sensu on the same machine and I can send email with the same smtp account and credentials. Here's the command output:

* Connected to pro1.mail.ovh.net (79.137.0.66) port 587 (#0)
< 220 pro1.mail.ovh.net Microsoft ESMTP MAIL Service ready at Wed, 11 Oct 2023 10:26:50 +0200
> EHLO mail.txt
< 250-pro1.mail.ovh.net Hello [176.191.46.127]
< 250-SIZE 104857600
< 250-PIPELINING
< 250-DSN
< 250-ENHANCEDSTATUSCODES
< 250-STARTTLS
< 250-AUTH GSSAPI NTLM
< 250-8BITMIME
< 250-BINARYMIME
< 250 CHUNKING
> STARTTLS
< 220 2.0.0 SMTP server ready
*  CAfile: /etc/ssl/certs/ca-certificates.crt
*  CApath: /etc/ssl/certs
* TLSv1.0 (OUT), TLS header, Certificate Status (22):
} [5 bytes data]
* TLSv1.3 (OUT), TLS handshake, Client hello (1):
} [512 bytes data]
* TLSv1.2 (IN), TLS header, Certificate Status (22):
{ [5 bytes data]
* TLSv1.3 (IN), TLS handshake, Server hello (2):
{ [85 bytes data]
* TLSv1.2 (IN), TLS handshake, Certificate (11):
{ [3433 bytes data]
* TLSv1.2 (IN), TLS handshake, Server key exchange (12):
{ [657 bytes data]
* TLSv1.2 (IN), TLS handshake, Server finished (14):
{ [4 bytes data]
* TLSv1.2 (OUT), TLS header, Certificate Status (22):
} [5 bytes data]
* TLSv1.2 (OUT), TLS handshake, Client key exchange (16):
} [138 bytes data]
* TLSv1.2 (OUT), TLS header, Finished (20):
} [5 bytes data]
* TLSv1.2 (OUT), TLS change cipher, Change cipher spec (1):
} [1 bytes data]
* TLSv1.2 (OUT), TLS header, Certificate Status (22):
} [5 bytes data]
* TLSv1.2 (OUT), TLS handshake, Finished (20):
} [16 bytes data]
* TLSv1.2 (IN), TLS header, Finished (20):
{ [5 bytes data]
* TLSv1.2 (IN), TLS header, Certificate Status (22):
{ [5 bytes data]
* TLSv1.2 (IN), TLS handshake, Finished (20):
{ [16 bytes data]
* SSL connection using TLSv1.2 / ECDHE-RSA-AES256-SHA384
* Server certificate:
*  subject: CN=pro1.mail.ovh.net
*  start date: Jun  7 00:00:00 2023 GMT
*  expire date: Jun  6 23:59:59 2024 GMT
*  subjectAltName: host "pro1.mail.ovh.net" matched cert's "pro1.mail.ovh.net"
*  issuer: C=GB; ST=Greater Manchester; L=Salford; O=Sectigo Limited; CN=Sectigo RSA Domain Validation Secure Server CA
*  SSL certificate verify ok.
* TLSv1.2 (OUT), TLS header, Supplemental data (23):
} [5 bytes data]
> EHLO mail.txt
* TLSv1.2 (IN), TLS header, Supplemental data (23):
{ [5 bytes data]
< 250-pro1.mail.ovh.net Hello [176.191.46.127]
< 250-SIZE 104857600
< 250-PIPELINING
< 250-DSN
< 250-ENHANCEDSTATUSCODES
< 250-AUTH GSSAPI NTLM LOGIN
< 250-8BITMIME
< 250-BINARYMIME
< 250 CHUNKING
* TLSv1.2 (OUT), TLS header, Supplemental data (23):
} [5 bytes data]
> AUTH GSSAPI
* TLSv1.2 (IN), TLS header, Supplemental data (23):
{ [5 bytes data]
< 334 GSSAPI supported
* gss_init_sec_context() failed: No credentials were supplied, or the credentials were unavailable or inaccessible. No Kerberos credentials available (default cache: FILE:/tmp/krb5cc_1000).
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
* Closing connection 0
* TLSv1.2 (OUT), TLS header, Unknown (21):
} [5 bytes data]
* TLSv1.2 (OUT), TLS alert, close notify (256):
} [2 bytes data]
curl: (94) An authentication function returned an error

I don't understand because I pass the --user 'xxxx@xxx.com:mypassword'. I even tried to remove the password from the command line, so curl prompt for it, but I have the same error message then...

curl -v --ssl-reqd  --url 'smtp://pro1.mail.ovh.net:587' --user 'username@gmail.com:password' --mail-from 'username@gmail.com' --mail-rcpt 'john@example.com' --upload-file mail.txt

for that you need mail.txt file in the folder where you launch curl with content like this:

From: "User Name" <username@gmail.com>
To: "John Smith" <john@example.com>
Subject: This is a test

Hi John,
I’m sending this mail with curl thru my gmail account.
Bye!

Please adjust the command and mail.txt file with your actual addresses. curl should produce verbose output, pls share that.

tcpdump -s 0 -w smtp.pcap -i INTERFACE_NAME host pro1.mail.ovh.net and tcp and port 587

STARTTLS has unencrypted phase of communications in the beginning, so we might see some clue there. You can send capture file to me in a private message.