Within a company, almost all discussion of issues or problems occurs through email, including very high level and/or legal matters. Email allows instantaneous distribution to any number of people, and it allows the sender complete control over the initial recipient group. Hard copies of letters or memoranda are much more difficult and time-consuming to distribute, and unauthorized persons often have access to hard copy material, so email is the preferred method for communication within a company.
Correspondence or reports to anyone outside a company can be, and often is, in both email and letter form. The email precedes the hard copy and usually confirms that a letter will follow. Most of the time, letter reports are written very formally and carry the signature of the highest level person involved. Emails can be generated by almost anyone who is involved with an issue or problem and has the authority to respond. Hard copy correspondence still carries slightly more weight than email counterparts.
From a practical perspective, probably 75%-85% of a company's day-to-day communication with outside parties is in the form of emails. It is rare these days that anything but highly sensitive material is distributed in the form of letters. There are issues, however, that are so important that the appropriate form is formal correspondence rather than email, but these issues or problems are often related to legal matters and are handled by either internal or external counsel.
Because a high percentage of external communications concerns problems that need to be resolved quickly, and the resolution communicated quickly, these almost always go by email, which is so common in the business world that any reader considers an email as a formal response equal to a response in letter format.
I hope that helps.