There is encoding problem in headers with central european characters.
Recipient address "Jan Mikuš mikus@itcomputers.cz" after encoding return "=?UTF-8?Q?Jan=20Miku=C5=A1=20mikus@itcomputers.cz?=" (right is "=?UTF-8?Q?Jan=20Miku=C5=A1?= mikus@itcomputers.cz") and Postfix this header encode to "=?UTF-8?Q?Jan=20Miku=C5=A1=20 mikus@itcomputers.cz,\n ?=@arachne.itcomputers.cz".