It's also possible that the quality of one's writing is affected by the quality of the relationship with the other. Spelling and grammar conjure up all sorts of memories and emotions from the school years of one's childhood. Your self-concept may ride on those memories. In the course of an e-mail relationship, those issues from the past may be stirred up. Writing isn't just a tool for developing the e-mail relationship.
Writing affects the relationship, and the relationship influences the quality of the writing.
Writing effectiveness changes as a result of what is happening in the ongoing e-mail encounter. Those changes reflect the developmental changes in the relationship. Compulsive people may construct highly organized, intellectualized messages with little emotional revelation. Histrionic people may show less concern about organization and much more for the emotions they express. Narcissists may write extremely long, rambling blocks of paragraphs.