Explore allowing only one authentication_mechanism on SmtpTransportBuilder

[craftytrickster]

Currently, the SmtpTransportBuilder has a field called authentication_mechanisms that allows for an Vec of mechanisms.

This caught my eye because it seemed like an unnecessary wasteful Vec allocation, but then I notice that you allow for multiple assignments to check them in priority order as seen here.

I cannot think of a scenario in which I would be targeting an email relay without being sure of how to authenticate. For example, if the relay server expects CramMd5 and that fails, it seems weird that I would also want to check for Plain, just in case it would magically go through. But that is what is allowed if I specify multiple mechanisms in the Vec. If Plain went through, even though the server was expecting CramMd5, it would actually be dangerous, because it would hide the authentication problem from me, since the emails would be sent successfully during the Plain fallback.