No one is going to stick to grammar rules here. Probably the reason for it is that very few of the penpals can type without looking onto the keys. So the person usually does not have an opportunity to see that is on the screen. In this case the text is typed rather slowly and one has to make it as simple as possible.
The letters may lack some letters or punctuation marks. Please take into account that it does not mean that the penpals do not respect each other or that they are absolutely illiterate. This only means that the person is not a professional in using the computer (we hope that you will agree that this can be forgiven)
Be ready that the first letters may be rather short. Sometimes they may even contain only one sentence. Once again we would like to stress that it does not mean that the intentions of your potential penpals are not serious. Apart from the problems in typing long texts. You should also keep in mind that correspondence through the internet is absolutely anonymous, the don't even know your e-mail address. Your potential penpals have no guarantees that you will decide to answer them.
Sometimes you can across very long first letters. It is most likely that these are standard letters which are sent everyone. It is absolutely natural. You are also free to write to as many people as you wish. You also have an advantage: the person who is writing to you is interested in you, and he has no guarantee that that he will get a response.
Probably you get very many letters and you have chosen the very men you are interested in. In this case the opportunity that you will still decide to answer the letter is rather vague. You should be understand that screening is usually made after the first three letters. If you are still interested in each other after that the correspondence usually becomes rather stable.