A mail letter fell into the place where you take lint out of the dryer and i cant get it out will it catch on?

Yes..it will most definitely catch fire! Do not use the dryer until you get the envelope out. You can usually remove the back of the dryer and get it out that way. If this is not an option, another way is to use a dryer vent brush ( one that you use to clean out dryer vents with) to push it out the vent connection to the outside. Another way but more expensive is to go to your local hardware store and get an electrician's fish line ( a line used to pull wires through conduit). Good Luck!