Hello, our mail server is running on CentOS. It has both postfix and ASSP installed.
When trying to access the email address outside the company, it allows me to receive but not send. It says it does not support authentication or something to that extent. I know this has something to do with the Anti-Spam SMTP Proxy Server. Unfortunately our manager was laid off and I'm no Linux expert. Thanks in advance.

First of all..
How are you trying to access the email externally? Are you using a webmail service or just connecting to smtp/pop directrly externally?
Have you looked at /var/log/maillog?

I'm accessing IMAP/SMTP directly.
This issue may already be resolved because I've had one successful test. I will update with more information when I'm sure.

ok no worries.

It turns out I just had to use port 25 instead of 587. Thank you

Unfortunately it looks like that didn't solve the issue. Although it works fine from my phone, the exact same settings on my laptop give errors. Receiving email works fine but sending does not. A fellow employee has the exact same issue sending emails from his phone.
Normally 143 is receiving and 587 is sending. I am using the latest version of Thunderbird.

This is the error I get:
When I switch the outgoing port to 25, it says:
When I try to send an email to myself:
When I try to send an email to someone else in the corporation:
All email addresses work fine inside the network.
Sorry if I seem like a noob. Our manager got laid off and I'm not a Linux expert by any means.
Thanks in advance.

When you try to send mail externally what does /var/log/maillog display?

I can't even find an entry for my email address. I've repeatedly opened the log file after I attempt to send one but an entry with my email address is nowhere to be found. There are a lot of "unknown" entries though:

Jul 3 11:02:22 mail postfix/smtpd[11544]: connect from unknown[127.0.0.1]
Jul 3 11:02:22 mail postfix/smtpd[11542]: 35BADBE2ED7: client=unknown[127.0.0.1]
Jul 3 11:02:22 mail postfix/smtpd[11542]: lost connection after RCPT from unknown[127.0.0.1]
Jul 3 11:02:22 mail postfix/smtpd[11542]: disconnect from unknown[127.0.0.1]
Jul 3 11:02:23 mail postfix/smtpd[11543]: connect from unknown[127.0.0.1]

Thanks in advance.

Well that sounds like the mail isn't even getting through to your mail server. Maybe a firewall is blocking the traffic? If you run a tcpdump on the mail server

$tcpdump port 25

and try to send some mail, do you see traffic?

Did you add your IP to /etc/mail/access ?

It responds with:
-bash: tcpdump: command not found

I don't have a /etc/mail/access file but I do have /etc/postfix/access
Would this be an internal or external IP address, and would this need a restart to postfix?

I cannot speak with authority on this as I have not run a postfix mail server. I am familiar with sendmail and run a sendmail server, however. In sendmail, you have to specify which IP addresses will be relaying through the server in /etc/mail/access. Any other IP addresses attempting to send mail through the server will be rejected. (edit) The address that you would add would be whatever address you are trying to send mail directly from.

---------- Post added 07-03-14 at 01:38 PM ----------

Sounds like you don't have /usr/sbin in your PATH. Call it directly.

Thank you everyone for your suggestions but I actually found what I need in the ASSP client interface. There is a section called (acceptAllMail) which is where I whitelist certain IP address for external usage. I just tested my laptop's IP address and it worked perfectly.

Obviously I don't want this system to be totally 100% open for security reasons. Since the majority of these people will be accessing their email outside via smartphones, does anyone know the IP address blocks for AT&T Mobility, Verizon Wireless, T-Mobile, Sprint, and MetroPCS? Thanks in advance.