postfix cannot send email
Solution 1:
Could be your ISP or hosting provider filtering outbound connections to port 25 to prevent botnet spam. Use
traceroute -n alt2.gmail-smtp-in.l.google.com
and/or
tcptraceroute -n alt2.gmail-smtp-in.l.google.com 25
to check which hop is blocking the connection.
Solution 2:
Gmail recommends connecting to smtp.gmail.com on port 587 for TLS/STARTTLS or 465 for SSL. http://support.google.com/mail/bin/answer.py?hl=en&answer=13287