Since no one really stated it in a way that's fully clear . . .

It has nothing to do with L vs T class. Those both will earn 100% mileage if booked directly through Delta.

If you purchase those tickets through another travel agent, they might be special versions of fares called "consolidator fares" and will only earn 25% mileage. It's a new rule in 2013. There's no easy way to know whether you'll get the full mileage or 25% mileage just by the single-letter fare class.

You'll need to either call Delta on your ticket after it's booked or else use a service like Expert Flyer (with some training) to identify whether or not it's a consolidator fare. (It's sort of a complicated process, but you can compare the 5-letter full fare class--starting with "L" or "T" in your case--from the detailed fare rules, with a list of published fares on Expert Flyer. If it's not in the Expert Flyer list, it's generally an unpublished consolidator fare.)

If your company will let you book your own tickets, buy them through Delta.com and you'll always get full mileage.