I digged a little bit further. The data being sent is

'Content-Type: multipart/mixed; boundary="===============1981330074035035012=="\r\nMIME-Version: 1.0\r\nFrom: rzrobot@seznam.cz\r\nTo: msladek@volny.cz\r\nSubject: =?utf-8?b?xb5sdcWlb3XEjWvDvSBrxa/FiA==?=\n\r\n--===============1981330074035035012==\r\nContent-Type: text/plain; charset="utf-8"\r\nMIME-Version: 1.0\r\nContent-Transfer-Encoding: base64\r\n\r\nw7pwxJtsIMSPw6FiZWxza8OpIMOzZHk=\n\r\n--===============1981330074035035012==--'

As you notice, there is a plain \n (without \r) after the subject (and all other places with base64), which might confuse seznam.