I'm running a Mac Pro, Early 2009, 2.66 GHz Quad-Core Intel Xeon, 20 GB of 1066 MHz DDR3, OS X 10.11.6 with MBM 8.6.3. I'm getting the dreaded 503 5.5.0 message in the header above. I'm not sure why, but it's possible that it's because the smtp address entered into MBM is using port 25 instead of the ports my ISP (smtp.mail.yahoo.com) says should be used, namely 465 or 587. Of course, this may not be the reasons. Any help proferred would be much appreciated.