cant get email from some domains

[massimiliano]

hello,
ive a working installation of roundcube, all its working fine but users are experiencing some problem cause cant get email from some domain.
the domain are public institutional domains (like city hall) so it is impossible are in some blacklist
where i can look to solve this problem ?

rarerly but still happen that from outside its impossible to send email to my server, the  sender get RCPT ERROR 550 user unknow .. but user exist

thank you