When adding multiple tags (for transports that support this), it's recommended to add multipleTagHeader's instead of a single header with a transport-specific way to separating.

$email->getHeaders()->add(newTagHeader('tag1'));$email->getHeaders()->add(newTagHeader('tag2'));// not$email->getHeaders()->add(newTagHeader('tag1,tag2'));

This fixes mailchimp to allow this. I've left the comma separated method intact as to not cause a BC break.