There isn't an easy answer. At agencies, correspondence can often be ignored without major consequence. But members of Congress neglect voters at great peril. Capitol Hill is a special case in the ongoing adaptation to communicating via e-mail. There are some real problems and perhaps some lessons for other parts of the government.
Congressional offices are vast mail machines.
A distressingly large percentage of the staff spends its time sorting and answering mail. If you write your own representative, you will get an answer. If you write to the wrong representative, your mail will be forwarded by a staff member all too happy to dump work onto someone else. Most congressional offices are now Internet-literate. But few are organized to handle e-mail.