You know, this is something that could be built as a "serverless" app for almost no cost using AWS services like SES, S3, Lambda, and DynamoDB. I'm sure that the VPS doesn't cost much, but it's a cost whether anyone is visiting the site or not.

Edit: Didn't mean to sound negative, so I hope it doesn't come across that way. Good job on building something! I've been struggling with that myself and these types of posts motivate me to just build something.