信 is a particular letter, 信件is more of a general noun for all kinds of letters. Pretty much like the difference between "a glass" and "glassware". eg 英文信件的格式是什么？ 不可随意拆看私人信件。

谢谢你!

信 , 信件 are pretty much the same. the first one is popular used one. take an example , 我写了一封信给我爸爸 信件， kina like sounds pretty pretty formal/professional AND old-fashioned so simply put. the first one , YOU CAN ALWAYS USE. the sec one , leave it out , please. hope that that helps