Can I include marketing content in transactional emails?

Yes, but with important limitations to maintain transactional status:

What is allowed:

Cross-sell recommendations related to the purchase. Referral program mentions. Social media links. General brand messaging.

What determines status:

The primary purpose test under CAN-SPAM: A reasonable recipient would consider the email primarily transactional, not marketing.

Best practices:

Transactional content should be above the fold and clearly primary. Marketing content should be secondary and proportionally small. Avoid promotional subject lines that obscure the transactional nature.

Risks of over-marketing:

Lose transactional exemptions. Increase complaints on critical emails. Damage the trust transactional emails depend on.

Transactional emails are opportunities, but handle them carefully. The recipient opened for information, not promotion. Respect that intent.