Wt

When a WApplication runs in a localized environment other than "en|English" then Wt's E-Mail system might produce non RFC 5322 compliant DATE headers.

This is because the translated non-english strings are being used for the DATE string in the E-Mail header, while RFC 5322/RFC 2822 expects only english day names.

This happens in src/Wt/Mail/Message.C :

void Message::write(std::ostream& out) const
{
...
if (!date_.isNull())
out << "Date: " << date_.toString("ddd, dd MMM yyyy HH:mm:ss Z") << "\r\n";
...
}

If WApplication runs in a locale other then English, then .tostring() will use the non-english localized strings (Wt.WDate.Mon to Wt.WDate.Sun) for the DATE header.

Several E-Mail providers will reject these E-Mails with reason:

INVALID_DATE Invalid Date: header (not RFC 2822)

It took me days and hours to find out why E-Mails from my application are constantly being rejected and what's wrong with the date headers :-/ For the meantime, I change WApplication::locale to 'en' immediately before Wt::Mail::Client::send and change it back to the users selected locale afterwards.

Thus:
This refers to Wt 4.10.0, independend of the current locale, Wt::Mail::Message always should produce english day names in the DATE header.