4.0.0

Improved encoding of email headers for key toString() and getEncodedHeaders() methods by using the new Symfony/Mime component.

Public API is largely unchanged, but there are two little-used methods removed, and remaining primary email headers have been given dedicated getters and setters so can no longer be set using addHeader(): Message-Id, (Origination) Date, Received, In-Reply-To and References.

See Change Log for full details.