In addition to using merge tags to personalize your message content, you can also use merge tags to customize some email headers. Currently, merge tags are supported in the from name, subject line, and body of an email only. Merge tags can’t be included in custom X- headers or the List-Unsubscribe header.
情報源: Can I use merge tags in my subject line or other headers? – Mandrill Knowledge Base
MailChimpの件名に名前以外のMerge Tagは使えるのか?という疑問に対して。結論、Yes。早速使う。