We've recently started adding DomainKeys signatures to some of the mail sent via mail.ru.ac.za.
DomainKeys provides a way for the recipients of e-mail to check whether the sender of a message is entitled to make use of the domain they're using, and to verify that the message has not been altered in transit. If forms part of what's known as identified Internet mail, which is an attempt to reduce the impact of unsolicited e-mail (SPAM). It is widely used by, for example, Yahoo! mail and Gmail.
Any mail sent from mail.ru.ac.za via authenticated submission (i.e. verified with a username and password) that has an envelope sender containing a Rhodes (@ru.ac.za or @campus.ru.ac.za) e-mail address will automatically be signed.
This change will be invisible to most users. If you don't understand the above, you can safely ignore this notice.