I'm trying to send an email to 14,000 recipients, and it keeps getting rejected at the payment stage. The credit card rejection error says "Referral." I know it's not the card and I've used it to send to smaller numbers of recipients. Is there a maximum I'm running up against? Thanks.
This is an issue with your card (or actually, with your card provider). A 'referral' message comes from your bank / provider, and basically means they need you to contact them and verify it is a legitimate transaction.
Form our side, unfortunately we can't do anything except submit the request as normal, so this is one you'll need to follow up with your bank on.
There isn't really a maximum list size in Campaign Monitor, but we do have an approval process you can see here: http://help.campaignmonitor.com/topic.aspx?t=95