We'll be upgrading the roundcube webmail today - this will mean some moments when it's not available - however, this should only be for a short amount of time.
SquirellMail will be unaffected.
The upgrade has been completed. The new features include:
SMTP delivery Status notifications (under the 'Setting' icon when composing a message)
Threaded Messages option ('Options' icon on next to the column titles)
Contact Groups (under the Address book, you can now create groups)