Lets say that i need to send about 5000 email with different content dailly from subscriber search criteria (ex : Job alert).
how the best way to achieve high performance for searching criteria in database and sending email and email provider not mark it as a spam.
The best way? In my opinion, the best way is to use an external service, such as MailChimp.com, that has an API you can use and send message through them.
This will greatly speed up your development time, and offload the message to a reputable, typically white-listed, provider.