https://web.archive.org/web/20150709...A-MTA-MDA.html
According to the diagram there, MDA is contacting with a MTA. See the arrow.
If on host2.uiuc.edu, this diagram is valid,
MTA -> MDA -> MUA
then shouldn't this diagram be valid on host1.uiuc.edu:
MUA -> MDA -> MTA
So, shouldn't that arrow be from MTA to MTA rather than from MDA to MTA?
MUA -> MDA -> MTA <-----> MTA -> MDA -> MUA