Problem using email client from command line

Hello,

This is the "scenario".

I have a Vmware network on which a number of servers (Domain controllers, administrative management servers etc.) are virtualized.

Connected to this network are physical devices such as clients, printers etc.

The entire infrastructure is connected to a managed HUB.

Between the HUB and Internet is a Sophos XG Firewall (116w) and a DSL Modem provided by the provider.

So far all is okay and the clients navigate correctly and also send and receive e-mail.

A management application is installed on an administrative virtual server that uses a small program (blat.exe) to send emails with attachments.

The management application compiles a string and run it, e.g.

blat mail_body_message.txt -to destination@email.com -subject “Email Object” -attach attachment.pdf -f sender@sender_email.com - server server_smtp -u username -pw password

At this point the command is not executed correctly and in the log I find

2023.01.18 08:43:09 (Wed)------------Start of Session-----------------
Blat v3.0.0 w/GSS encryption (build : Feb 17 2012 17:46:41)
<<<getline<<< 220 SMTP ESMTP ready
>>>putline>>> EHLO cg-server.domain.it
<<<getline<<< 250-SMTP Hello cg-server.domain.lcl [192.168.1.110]
<<<getline<<< 250-SIZE
<<<getline<<< 250-8BITMIME
<<<getline<<< 250-PIPELINING
<<<getline<<< 250-PIPE_CONNECT
<<<getline<<< 250-CHUNKING
<<<getline<<< 250-STARTTLS
<<<getline<<< 250 HELP
Sending mail_body_message.txt  to destination@email.com
Subject: “Email Object”
Login name is sender@sender_email.com
>>>putline>>> MAIL FROM:<sender@sender_email.com>
<<<getline<<< 250 OK
>>>putline>>> RCPT TO:<destination@email.com>
<<<getline<<< 550 Relay not permitted
The SMTP server does not like the name destination@email.com.
Have you set the 'To:' field correctly, or do you need authorization (-u/-pw) ?
The SMTP server response was -> 550 Relay not permitted
>>>putline>>> QUIT
Error: Connection to server was dropped.
2023.01.18 08:43:11 (Wed)-------------End of Session------------------

To check if it is a firewall problem, I disconnected the Sophos XG and connected the network directly to the DSL modem.

Everything is working correctly.

Can I be helped to solve the problem?

Thank You and Best Regards.

Gaetano