I have created a newsletter application in .Net for one of my clients. The database is hosted on a server and application is also hosted on another server. My question is, when my client will login and create a newsletter (an html file is created). And then send newsletter email to about 50,000 email addresses, what memory resources will be used?
P.S. My application loads all email addresses in a server variable and then loop through each value and send's email.
To create and send 50.000 emails, i think you don't have to worry about memory resources.
But you forgot the most important part, the mail Gateway. This one needs to be configured correctly or otherwise some email providers will treat your mails as spam. To be sure you have to Setup techniques like SPF, DKIM and DMARC.. which is not an easy thing. Therefore I suggest, you take a look at third party services that are specialized for mass email delivery, than creating the infrastructure on your own. In addition to an easy API, they provide you with Dashboards and lots of useful Information (which emails are delivered/opened/clicked/...)
There are a lot providers, just google for "email delivery Service" and you will find them. They have different pricing models, just see which one fits your needs. Some of them are: