SMTP with Postfix only works internally.

I've got my email server set up with Postfix/Dovecot using the documentation from Linode. I've installed roundcube as a web interface and it can send and receive email without trouble. However, when I try to use an external client, I can only receive mail.

So my first thought was the firewall. Here's iptables -L:

Chain INPUT (policy ACCEPT)
target     prot opt source               destination         
ACCEPT     all  --  anywhere             anywhere            
REJECT     all  --  anywhere             127.0.0.0/8         reject-with icmp-port-unreachable 
ACCEPT     all  --  anywhere             anywhere            state RELATED,ESTABLISHED 
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:www 
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:https 
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:imap2 
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:585 
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:imaps 
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:2025 
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:smtp 
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:ssmtp 
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:submission 
ACCEPT     tcp  --  anywhere             anywhere            state NEW tcp dpt:ssh 
ACCEPT     icmp --  anywhere             anywhere            icmp echo-request 
LOG        all  --  anywhere             anywhere            limit: avg 5/min burst 5 LOG level debug prefix `iptables denied: ' 
REJECT     all  --  anywhere             anywhere            reject-with icmp-port-unreachable 

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         
REJECT     all  --  anywhere             anywhere            reject-with icmp-port-unreachable 

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination         
ACCEPT     all  --  anywhere             anywhere   

Any obvious mistakes?

And here's the output of netstat -lntp:

Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 0.0.0.0:993             0.0.0.0:*               LISTEN      10153/dovecot   
tcp        0      0 0.0.0.0:995             0.0.0.0:*               LISTEN      10153/dovecot   
tcp        0      0 127.0.0.1:44069         0.0.0.0:*               LISTEN      23261/current   
tcp        0      0 127.0.0.1:54470         0.0.0.0:*               LISTEN      27868/current   
tcp        0      0 127.0.0.1:3306          0.0.0.0:*               LISTEN      6765/mysqld     
tcp        0      0 127.0.0.1:40718         0.0.0.0:*               LISTEN      18128/current   
tcp        0      0 0.0.0.0:110             0.0.0.0:*               LISTEN      10153/dovecot   
tcp        0      0 0.0.0.0:143             0.0.0.0:*               LISTEN      10153/dovecot   
tcp        0      0 127.0.0.1:39154         0.0.0.0:*               LISTEN      19565/current   
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      2084/sshd       
tcp        0      0 0.0.0.0:25              0.0.0.0:*               LISTEN      16962/master    
tcp6       0      0 :::80                   :::*                    LISTEN      14926/apache2   
tcp6       0      0 :::22                   :::*                    LISTEN      2084/sshd   

So it looks like something is listening on port 25, right?

Thanks for any help you can offer.