In the Global Settings you can set the two properties 'SMTPDateHeaderFormat' and 'SMTPDateHeaderTimeZone' to the desired format for your particular region, you must have Administrator access to change these settings.
For example: Adelaide, Australia has a UTC time of +09:30 and the settings should be:
SMTPDateHeaderFormat: ddd, dd MMMMMMMMM yyyy HH:mm:ss
SMTPDateHeaderTimeZone: +0930
NOTE: When adding the time zone in the Global Settings ensure you add the extra whitespace needed before the + sign.