Major changes:
- Automatic opportunistic TLS encryption even if you don't specify
SMTPSecure = 'tls' - Parsing of SMTP server capabilities
- The html2text class in extras had to be removed for licence reasons, inject your own converter
- Better standards compliance on timeouts, encoding, line length, DKIM, MIME structure
- Allow additional certs for S/MIME signing
- More languages - now up to 47!
- Provide stream context options to allow finer control of SSL options, such as allowing self-signed certs
- EasyPeasyICS much improved
- Better error reporting when individual messsages in a group fail, more detail on SMTP errors
- Improved checking for missing extensions (e.g. openssl)
- Improved composer config and docs