When attempting to send an email to both lists, I received an error message:

Then I tried sending the message to the smaller list alone and received the same message.

Update: Received an email that I was blocked from sending (see below). Entered the captcha and got unblocked but I still can't send the email to my category list of 220, which should have gone out this morning. I see nothing about a limit for category lists - what's the point of having them if you can't use them?