Email is so convenient that we tend to forget that it has its limits, and while rapid, it is not instantaneous. But, it is essential. The trademark side of the USPTO won’t let you keep the cheaper TEAS-Plus status for an application unless you keep an authorized email address. Be aware, however, that the TMEP has an entire section, 304, which governs what communications are, and are not, acceptable via email, and such essentials as date of receipt and certificates of transmission. Check it out. Trademark Manual of Examining Procedure (TMEP) - January 2015