Reading the godoc:

    func (h Header) Get(key string) string

Get gets the first value associated with the given key. If there are no values associated with the key, Get returns "".

It's not immediately obvious that key is not case sensitive. As it turns out, Get("From") and Get("from") mean the same because internally this all goes through https://golang.org/pkg/net/textproto/#CanonicalMIMEHeaderKey.

Perhaps this is just me not being used to MIME Headers being canonical in this way. But I think a sentence like "note that key is case insensitive as it is canonicalized" would help some people. The net/mail godoc does not mention MIME headers anywhere either.

Perhaps this should go in net/textproto too, even though that's much closer to the actual CanonicalMIMEHeaderKey func.