I'm using Mandrill-api for Nodejs.
When creating a message for multiple recipients I can create merge_vars for each one of them, but is there a way to give each recipient a different subject. Im sending invoices, and would like to have a subject like: "Invoice X", where X is a invoice number =P
You should be able to use merge tags in the subject line, and then pass recipient-specific subject lines. A lot of people use this to personalize the subject line for each recipient with their name, for example. So you could pass the subject as Invoice *|INVOICENO|* - if you're already putting the invoice number in the email content with a merge tag, just use the same merge tag in the subject.