Index Strategies on Text or NVARCHAR(MAX) Fields
Why do you think anything but a scan should be used to pull back all the data? A full-text index won't really help - that helps you search those columns, but if you're just returning all the data (for any variety of WHERE clauses) then there's no shortcut to reading all of the data. Can I ask why a to_addr, which is presumably limited to ~320 characters by the SMTP standards (depending on which standard you believe), contains data > 4000 characters?

A lot of people think that a scan is bad. If you need to return a large amount of data, then often a clustered index scan will be used. Your where clause may lead to seeks being used to locate the rows to return, but a seek isn't going to work where the data in that column is that large. Are you just seeing a scan in the execution plan and assuming that must be the problem?