Hi {{contact.first-name}},
I hope you're having a great day! I came across your website while researching [topic] and was impressed by the quality of your content.
I noticed that you haven't covered [subtopic] yet, which is why I wanted to reach out. I recently published a comprehensive guide on that exact topic and thought it might be a good fit for your audience.
Would you be open to featuring it as a guest post on [website name]? I believe it could add value to your site while also helping to increase both our online visibility.
Looking forward to hearing from you!
Warm regards,
[your name]