Due to an unfortunate oversight on our end the SSL certificates for our DE mail servers expired earlier today at around 09:30 UTC, causing certificate warnings to appear in email client software using DE mailboxes and in our DE webmail system.
Renewed certificates were rolled out approximately 1 hour later and there have been no issues since that time (~10:30 UTC). If you continue to see a certificate warning when accessing DE email systems then please restart your software and/or your device.