You can add an array of strings to tag the message with. Stats are accumulated using tags, though we only store the first 100 we see, so this should not be unique or change frequently. Tags should be 50 characters or less. Any tags starting with an underscore are reserved for internal use and will cause errors.
To tag your message sent via SMTP request you need to add a custom header
|X-Tags||Add tags to the outgoing message||Array of strings in JSON-format shielded with backslashes, each string being a tag to apply to the message.
No more than 50 characters per tag and 100 tags per account
|X-Tags: [\”tag1\”, \”tag2\”]
|X-Message-Priority||Add priority tag to the outgoing emails to avoid delivery delays for important messages due to large promo campaigns in queue||Priority 1 is default value.
You can have up to 5 priority levels.
Priority level over 5 will be rounded to 5.