I have recently come across a little issue as well. Not sure what's going on, but when someone in our league responds to all for a trade that was made...it seems to leave some people off the list. I have checked, and everyone in my league has the box checked to automaticly recieve trade offers and results. I'm not sure if this is a problem with the sending of the trade result or if it's in peoples email clients.
MFL support is great, but all computer nerds (myself included) don't like to be told there is a bug in their code. You have to go out of your way to show them that you have a legit issue and it's not user error.
First thing I would do is make sure everyone is setup to receive email in plain text format only. For this type of communication, html formatted messages are not really all that useful and it is one of many criteria that spam filters will use to "grade" the message. Plain text has a better chance of getting through filters.There is a second reason why this will help.
If I recall correctly, they will actually send out two messages to the end users. One to those who receive plain text, one to those that receive html formatted messages.
If you respond all to a message, it will only include those who were in the same group as you. I hope that makes sense. Just in case it doesn't, I'll give an example.
Let's say you have a twelve team league. Eight of the owners are setup to receive plain text. Four are setup to receive HTML.
You propose a trade and it is accepted. The server on their end now sends out two notices. The first goes to the eight owners setup for plain text. The other goes to the four owners that are setup for html. Two seperate distribution lists. The obvious problem here is that if you want to respond to the notice, you will only reach those who were included in the group you belong to.
By changing all users to receive plain text, they should all be included in one group meaning only one message is sent out. You could also switch everyone to html formatted and achieve the same effect, but my preference for simple messages like these is plain text.