手紙
手紙 – tegami. Specifically refers to physical letters that you can send to your favorite seiyuu via their agency (mail it to an address with a prescribed format, usually the department and the seiyuu name) or through present boxes during events. This does not apply to mails sent for programs (see mēru).
As one would expect, seiyuu can’t personally respond to letters but it is openly appreciated, some going as far as encouraging fans to send email to programs they host or appear as guests in so they can respond to it directly.
Handwritten letters are ideal but if you can’t communicate or write in Japanese, you can handwrite it in English then have it translated and printed out as part of the letter. When and how the seiyuu receives it is on a case-by-case basis because management has to screen them — some get to read almost immediately after the show, some on their way back from an out-of-town assignment, some maybe weeks even months afterward especially if there are too many. If you included a gift, make sure to mention and describe it in the letter so the addressee would know since those items tend to be screened separately.