No subject. Instead, reply to existing email chain.
Hi [hiring manager],
Thank you again for your generous offer to join [company]. Over the past weeks, I️ have truly enjoyed getting to know you, your team, and the company as a whole. Your [company's mission] is inspiring and it's potential is without a doubt exciting.
After much consideration, I️ am sorry to let you know that I️ must decline your offer. Please know that this decision was not so much a reflection of my belief in [company]'s mission, but rather, [your reason: the salary, the equity, a competing offer, the culture, etc.]. This was not an easy decision and I️ hope you can understand my reasoning.
Please let me know if you need anything else from me to close out this process. I️ am also happy to jump on a quick call if you'd like more insight into my decision making process. I️ look forward to watching [company] continue to grow, and if there is anything I️ can do to help, please do not hesitate to reach out!
Best,
[your name]