Enforcing connections to the ssl smtp server

I'll try to ask for help in the more frequented French area, excuse my poor English (I'm actually Italian)

I get problems sending e-mails from the command line, both Windows and *Nix
Basically, for an unknown reason, ssl0.ovh.net does not always "route" traffic to the SMTP server (!)

Perhaps it is better to give an example (smtp-cli, *nix)
We need to enforce -ssl, port 465, and plain auth

smtp-cli -verbose --auth-plain -ssl -server=ssl0.ovh.net --port 465 -4 -user=log@francocorbelli.it -pass=***mygoodpassword*** -from=log@francocorbelli.it -to dottcorbelli@gmail.com -subject "prova OVH"

The result

Connection from 192.168.1.254:34707 to 193.70.18.144:465
Starting SMTP/SSL...
Using cipher: ECDHE-RSA-AES256-GCM-SHA384
Subject Name: /CN=ns0.ovh.net
Issuer Name: /C=GB/ST=Greater Manchester/L=Salford/O=Sectigo Limited/CN=Sectigo RSA Domain Validation Secure Server CA
[220] 'GARM-110S004 Thursday, May 4, 2023'
> HELO localhost
*******************************************
**[250] 'OVH Mail Proxy'**
*******************************************
> MAIL FROM:
[530] '5.7.1 Client was not authenticated'
MAIL FROM failed: '530 5.7.1 Client was not authenticated'


Please look at the answer: it is [250] 'OVH Mail Proxy

OK, let's try with curl a quick-and-dirty.
This WORKS

curl -v --ssl-reqd "smtps://ssl0.ovh.net:465" --mail-from log@francocorbelli.it --mail-rcpt log@francocorbelli.it --user 'log@francocorbelli.it:****mypassword****' --insecure --upload-file mail.txt


root@aserver:~/script # curl -v --ssl-reqd "smtps://ssl0.ovh.net:465" --mail-from log@francocorbelli.it --mail-rcpt log@francocorbelli.it --user 'log@francocorbelli.it:LaMiaBellaPassword23' --insecure --upload-file mail.txt

(...)* Connected to ssl0.ovh.net (193.70.18.144) port 465 (#0)
* Cipher selection: ALL:!EXPORT:!EXPORT40:!EXPORT56:!aNULL:!LOW:!RC4:@STRENGTH
* successfully set certificate verify locations:
* CAfile: /usr/local/share/certs/ca-root-nss.crt
CApath: none
* TLSv1.2 (OUT), TLS header, Certificate Status (22):
} [5 bytes data]
(...)* SSL certificate verify ok.
{ [5 bytes data]
< 220 GARM-99G003 Thursday, May 4, 2023
} [5 bytes data]
> EHLO mail.txt
{ [5 bytes data]
< **250-OVH SMTP PROXY Hello**
*****************************************
{ [5 bytes data]
< 250-SIZE 104857600
{ [5 bytes data]
< 250-ENHANCEDSTATUSCODES
{ [5 bytes data]
< 250-AUTH LOGIN PLAIN
{ [5 bytes data]
< 250-AUTH=LOGIN PLAIN
{ [5 bytes data]
< 250 8BITMIME
} [5 bytes data]
> AUTH PLAIN
{ [5 bytes data]
< 334
} [5 bytes data]
(...)

In this successfully test we get a 250-OVH SMTP PROXY Hello

Same thing logging Thunderbird (ak: OK, we get the SMTP server)
(...)
_mailnews.smtp: Sending message SmtpService.jsm:85:18_
_mailnews.smtp: Connecting to smtp://ssl0.ovh.net:465 SmtpClient.jsm:119:17_
_mailnews.smtp: Connected SmtpClient.jsm:387:17_
_mailnews.smtp: S: 220 GARM-97G002 Thursday, May 4, 2023_

_SmtpClient.jsm:413:17_
_mailnews.smtp: C: EHLO [192.168.11.83] SmtpClient.jsm:590:19_
_mailnews.smtp: S: **250-OVH SMTP PROXY Hello**_
(...)

BUT problem on Windows, using another client (no SMTP server)

_Connecting to SMTP server: ssl0.ovh.net at Port: 465_
_Connection timeout: 5 secs_
_Forcing to use IPv4 address of SMTP server_
_> libmsock: using getaddrinfo_
_> AF_INET IPv4_
_ IP address: 193.70.18.144:465_
_> EINPROGRESS=10036,EWOULDBLOCK=10035_
_> connect(): socket=432,rc=-1, errno=10035_
_> Try socket 432_
_Cipher: ECDHE-RSA-AES256-SHA384_
_Certificate information:_
_Subject: /CN=ns0.ovh.net_
_Issuer: /C=GB/ST=Greater Manchester/L=Salford/O=Sectigo Limited/CN=Sectigo RSA Domain Validation Secure Server CA_
_[S] 220 GARM-101G004 Thursday, May 4, 2023_
_[C] HELO localhost_
_**[S] 250 OVH Mail Proxy**_
_[C] QUIT_
_[S] 221 Service closing transmission channel_
_C:\stor\bin>_


I think the key is here: **250-OVH SMTP PROXY Hello** (good) vs **[250] 'OVH Mail Proxy'** (bad)

I believe it is the OVH server that decides whether it is an SMTP session or not.
But ...how?

Thanks to all answers