The current implementation of blogs.Entry.send_to_list asks for a mailing list and then calls ar.send_email() for each member of the list. If the list has 1000 members, this will cause a browser timeout. If one member has an invalid email address, there will be a traceback and some members will have received the email and others not, and there is no way to see who received it and who didn't.

Add a model "Mailing" or "Sending" in the lists plugin. The send_to_list action just creates a new Mailing. And then a background task that actually sends the emails and keeps track of failures and successes.