Only people with valid Connect accounts can send email via Connect servers. If you do not have a Connect email account but have setup your mobile device and/or IMAP client to send email through the Connect server then you need to be aware of the immediate restriction being implemented:
What is happening? Sending email via Connect email servers is being restricted to those with Connect accounts. This new restriction may require some non-Connect (e.g. mailservices) users to review their local email settings.
Update - August 28 at 9:35 am: This issue was resolved late last night.
The central email service (mailservices) continues to be affected by blacklisting by ProofPoint (ProofPoint provides a blacklisting service that tracks and publishes lists of servers that have been detected as sources of SPAM. Because many UW credentials have been compromised, mailservices is now on the ProofPoint list).
Over the last two days, a number of compromised accounts were used to send SPAM through central email servers. While the compromised accounts were detected and disabled, the compromises have resulted in the mail servers being blacklisted by a number of sites. IST is working to get the servers off these blacklists.