SMTP won't authenticate (password error) but POP3 does.

My e-mail inbound e-mail (POP3) works fine in Thunderbird, but the outgoing e-mail (SMTP) gives me a password error.

I installed Virtualmin on an Ubuntu 18.04 server and added a domain (virtual server). I added an e-mail account to this domain and Virtualmin shows the IMAP/POP3/FTP login as user@domain.tld. In Webmin/System/Users and Groups I have two users for this e-mail account: user@domain.tld and user-domain.tld (with a hyphen instead of @).

I set up the account in Thunderbird with mail.domain.tld on port 995 for POP3 and mail.domain.tld on port 465 for SMTP and the user “user@domain.tld”. The POP3 works fine but the SMTP gives a password error. I tried changing the user for the SMTP to user-domain.tld, but I still get the error. What do I need to do to get the SMTP to authenticate?