Building An Import Process That Checks For Duplicates?

Using ASP.NET, I'm building an admin tool that requires a function to import a list of email addresses. Upon uploading the file, I want to check for existing records for any of the email addresses supplied. For non-existing email addresses, I would create them using my DAO.

Basically I want to:

Receive list of emails
Retrieve data for existing emails
Create data for new emails in db
Return full data for all emails in list.

Since I want to know which of the emails exist up front, my first thought was to query the table for all records WHERE Email IN ('Email001FromFile', 'Email002FromFile', 'etc...') but the list could potentially contain thousands of email addresses, and I'm not certain supplying that many email addresses to the IN operator would be a good idea.

I also thought about looping through the list and checking for a record for each email, but that would potentially generate far too many queries.

My next thought was to generate a temp table to hold the list and modify the IN clause to use the temp table, rather than an explicit list of items, but that would require I execute SQL or a stored procedure directly, which I'm not inclined to do since I'm using NHibernate to access my DB.

Though I am using ASP.NET (C#) and NHibernate