Email Node - Login fails on some servers

I have two servers on which I run Node Red (with docker), and I'm trying to send an email from each to the same recipients, with the same account and credentails.
On one of the servers I have two Node Red containers, both send emails perfectly (again, using the same account and credentials), but on the other server I get the following error:
"Error: Invalid login: 535 5.7.3 Authentication unsuccessful"

Secure connection and TLS are disabled, because this is the way it should be configured.
Any idea why it doesn't work on just one server?