I have seen issues with Comcast greylisting then blocking large volume domains. I am going to try to make this configuration to see if this helps. What I believe happens is they greylist and then on retry all the messages are in order by host which makes Comcast see a much large queue all at once which then makes them block due to quantity. I believe this because our normal quantities are not an unacceptable range based on their site and our senderscore. I believe we just drive it higher based on hold messages all going out in order of host which seems to be the default of new versions of sendmail.
define(`confQUEUE_SORT_ORDER', `Host')dnl
define(`confQUEUE_SORT_ORDER', `Time')dnl