*) Can you reach aspmx.l.google.com on your server? e.g. ping, host, dig, etc.
*) Can you telnet to Gmail port 25? For example:$ telnet 74.125.25.27 25 Trying 74.125.25.27... Connected to pa-in-f27.1e100.net. Escape character is '^]'. 220 mx.google.com ESMTP g9si14080014pac.68 - gsmtp
ping aspmx.l.google.com
PING aspmx.l.google.com (74.125.129.27) 56(84) bytes of data.
64 bytes from pd-in-f27.1e100.net (74.125.129.27): icmp_seq=1 ttl=45 time=195 ms
64 bytes from pd-in-f27.1e100.net (74.125.129.27): icmp_seq=2 ttl=45 time=201 ms
64 bytes from pd-in-f27.1e100.net (74.125.129.27): icmp_seq=3 ttl=45 time=199 ms
64 bytes from pd-in-f27.1e100.net (74.125.129.27): icmp_seq=4 ttl=45 time=195 ms
64 bytes from pd-in-f27.1e100.net (74.125.129.27): icmp_seq=5 ttl=45 time=198 ms
64 bytes from pd-in-f27.1e100.net (74.125.129.27): icmp_seq=6 ttl=45 time=202 ms
64 bytes from pd-in-f27.1e100.net (74.125.129.27): icmp_seq=7 ttl=45 time=198 ms
64 bytes from pd-in-f27.1e100.net (74.125.129.27): icmp_seq=8 ttl=45 time=199 ms
64 bytes from pd-in-f27.1e100.net (74.125.129.27): icmp_seq=9 ttl=45 time=198 ms
64 bytes from pd-in-f27.1e100.net (74.125.129.27): icmp_seq=10 ttl=45 time=198 ms
64 bytes from pd-in-f27.1e100.net (74.125.129.27): icmp_seq=11 ttl=45 time=201 ms
64 bytes from pd-in-f27.1e100.net (74.125.129.27): icmp_seq=12 ttl=45 time=201 ms
64 bytes from pd-in-f27.1e100.net (74.125.129.27): icmp_seq=13 ttl=45 time=201 ms
64 bytes from pd-in-f27.1e100.net (74.125.129.27): icmp_seq=14 ttl=45 time=201 ms
64 bytes from pd-in-f27.1e100.net (74.125.129.27): icmp_seq=15 ttl=45 time=195 ms
64 bytes from pd-in-f27.1e100.net (74.125.129.27): icmp_seq=16 ttl=45 time=201 ms
64 bytes from pd-in-f27.1e100.net (74.125.129.27): icmp_seq=17 ttl=45 time=197 ms
64 bytes from pd-in-f27.1e100.net (74.125.129.27): icmp_seq=18 ttl=45 time=202 ms
64 bytes from pd-in-f27.1e100.net (74.125.129.27): icmp_seq=19 ttl=45 time=200 ms
^C
--- aspmx.l.google.com ping statistics ---
19 packets transmitted, 19 received, 0% packet loss, time 18608ms
rtt min/avg/max/mdev = 195.027/199.569/202.863/2.316 ms
[root@mail ~]# telnet 74.125.25.27 25
Trying 74.125.25.27...
telnet: connect to address 74.125.25.27: Connection timed out
No it times out 
But the same for yahoo :
telnet 106.10.166.54 25
Trying 106.10.166.54...
Connected to 106.10.166.54.
Escape character is '^]'.
220 mta1153.mail.sg3.yahoo.com ESMTP YSmtpProxy service ready
Connection closed by foreign host.