Letter to friend:

It is permitted to write a friendly letter to an acquaintance on Chol Hamoed.[1] Nevertheless, one is to write the first line slightly slanted.[2]

____________________________________

[1] Bach; Taz; Elya Raba; M”B 545:35; See Michaber 545:5 regarding Sheilas Shalom and Prakmatya; M”B 545:4 and 30

Other Opinions: Some Poskim rule that the custom is not to write on Chol Hamoed even in regular script [even if it involves a need for the Moed, if it is not a case of loss or the need of the public] [Rama 545:5] unless one writes all the lines in a slant. [M”A 545:21; M”B 545:34] 

[2] M”B 545:35 in name of Peri Megadim