Issues with authentication SMTP mail in Virtualmin

SYSTEM INFORMATION
OS type and version: Ubuntu Linux 20.04.2
Webmin version: 1.981
Virtualmin version: 6.17-3
Related products version: RECOMMENDED

Hello there,
I hope someone can help me with this. I am totally new to Virtualmin.
I successfully created a new virtual server, linked it to a legitimate domain name, updated the DNS, and obtained the SSL with Let’s Encrypt. The domain is argiano.live
I created a new user for email purposes: info@argiano.live
Set up the email and password and quota.
When I configure the mail client (Mozilla Thunderbird) with the parameters mail.argiano.live and the authentication (passwords etc. both for IMAP and SMTP) it asks me again for password for the SMTP.
Never had this problem before and I think it’s the server that does not authenticate correctly.
I created another virtual server and same thing. In the past I have created tons of virtual servers and everything was OK.
Can anyone help me with this? Any suggestions?
Thank you in advance!