There are some of the connections providers that don't admit SMTP servers use on the port 25, at the moment, we only know 4: Wind, Tre, Vodafone and Teletu.
<<< To solve the problem you have to replace the port 25 with port 587 >>>
Another solution is using providers SMTP servers:
TRE (Business): smtp.tre.biz
If you still can't send email, contact us.