Unfortunately, based on the available documentation, the HAProxy mailers do not support authentication (any kind of authentication). However, at least AWS, the cloud based email services do require at least username / password authentication. Thus using AWS or Azure as direct SMTP servers for HAProxy is not possible.
I investigated exactly this issue a few weeks ago, and found no simple or safe way to do this… Most suggest setting-up Postfix, accepting unauthenticated email only from localhost, and forwarding it through the cloud provider SMTP endpoint (with authentication).
Another close enough alternative would be
nullmailer ([untroubled.org: nullmailer] or [nullmailer - Debian Wiki]) however as it was not available on openSUSE I didn’t try it.
Alternatively, and I’ve thought of this but didn’t manage to implement it yet, one could implement a very simple SMTP server in Go / Python / something-else that just speaks enough SMTP to make HAProxy happy, and then take the email payload and forward it with the help of a normal SMTP library for that language.