In sogo version 5 we have an option to write subject manually for vacation message. The subject is taken from webinterface and written to sieve script directly, as it is. It's all well if we use latin-1 characters, but when we want to use umlauts or cirillic special characters that gives us some trouble.
When script generates an email with such subject, in order to process it correctly postfix should propose SMTPUTF8 extension. I'm not sure if a large variety of email servers in the internet already so brilliant to flawlesly process utf-8 characters in headers without base64 encoding, but it's still pretty dangerous and unreliable. Means not every mail server is capable of accepting such autoreply.
I propose to stick to RFC 1342 and encode subject accordingly. For example, here what we have now:

if allof ( not exists ["list-help", "list-unsubscribe", "list-subscribe", "list-owner", "list-post", "list-archive", "list-id", "Mailing-List"], not header :comparator "i;ascii-casemap" :is "Precedence" ["list", "bulk", "junk"], not header :comparator "i;ascii-casemap" :matches "To" "Multiple recipients of*" ) { vacation :days 7 :subject "I'm on vacation until 13.06.2018 Äpfel УЁ їій" :addresses ["pavel.momot@ua.it-workbench.com"] text:
Hello. I'm currently on vacation and will respond to your letter after 13.06.2018. Thank you.

And this is how it should be in order to work (with base64 encoding, or you could use quoted-printable instead):

if allof ( not exists ["list-help", "list-unsubscribe", "list-subscribe", "list-owner", "list-post", "list-archive", "list-id", "Mailing-List"], not header :comparator "i;ascii-casemap" :is "Precedence" ["list", "bulk", "junk"], not header :comparator "i;ascii-casemap" :matches "To" "Multiple recipients of*" ) { vacation :days 1 :subject "=?utf-8?B?SSdtIG9uIHZhY2F0aW9uIHVudGlsIDEzLjA2LjIwMTggw4RwZmVsINCj0IEg0ZfRltC5?=" :addresses ["pavel.momot@ua.it-workbench.com", "admin@ua.it-workbench.com"] text:
Hello. I'm currently on vacation and will respond to your letter after 13.06.2018. Thank you.