According to domain regulation sub-paragraph 3.2 , aside from "-" and numbers (0-9) only Estonian alphabet letters can be used in a .ee domain name.

Estonian alphabet includes the following letters:

a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, š, z, ž, t, u, v, w, õ, ä, ö, ü, x, y

With the introduction of IDN domain names from July 13 2011 following IDN characters can be used in a .ee domain name after conversion to punycode:

õ – latin small letter O with tilde (U+00F5)
ä – latin small letter A with diaeresis(U+00E4)
ö – latin small letter O with diaeresis (U+00F6)
ü – latin small letter U with diaeresis (U+00FC)
š – latin small letter S with caron (U+0161)
ž – latin small letter Z with caron (U+017E)