waczny
It will entirely depend on your use case and volume of data. This can vary depending on many different things. Like if you accept attachments, the average size of the attachments, the emails headers, the email content, etc. Typically, text data like headers, content, etc. is very minimal however providers like MS LOVE adding so much crap to their emails that sending a simple "hello" email will contain many, many HTML tags, etc.
The best course of action is to give your server enough storage/resources to start off and then do an evaluation after a certain time frame. This way you have the best data possible for your use case to make a better informed decision.
Additionally, you can use the Filesystem or S3 plugin for attachments to offload the storage from the database so the database stays as small as possible for better performance, minimal backups, etc.
Cheers.
