DKIM Signature will usualy be provided working out of the box, as soon as you add a maildomain over hestia - in case you’re using hestia as dns server. If not, you’ll need to add the dkim txt records to your provider.
Also it will probaly not be an issue of DKIM only - we’ve aswell problems sending mails to providers like google or microsoft - even when I’ve got two long trusted ips which I send only valid traffic over it, I’ll get “spamboxed” from time to time.