In order to ensure quality of email sending, you must set the TXT record to officially declare which servers are authorized to send emails on behalf of the domain.
In ISPConfig control panel, DNS, create the TXT record as follows:
nomedominio.est. (Note the very important end point)
v = mx spf1 to: ms1.acsitalia.net to: ms4.acsitalia.net to: ms5.acsitalia.net to: sbg.acsitalia.net to: sbg2.acsitalia.net -all