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Also,i think i might be blacklisted on some domains,my mail server does not have reverse DNS and sometimes the Recipient blacklists my company's domain,is there any way to avoid this,i had read somewhere about implementing SMARTHOST in Sendmail but i have no idea how to implement it.
Please shed some light on this....
Also Please help with the 'Service Unavailable' ERROR.
Service Unavailable = service is unavailable. If you read the logs, you'll see that you contact the server, but the mail is not being delivered (could be miryad of reasons, from settings errors, maintenance downtime, messed up system, and so on). Try to contact the postmaster on the remote host to see what's going on..
Regarding your second question, the reverse DNS stuff, it's a countermeasure against spam implemented by many ISP which will deny mail delivery by servers that do not have a reverse DNS record (PTR record), which most spammy hosts/zombies lack. If you have a static IP address, try to contact your ISP and ask them for a PTR record assigment for your IP, preferably your hostname.
I'm a postmaster myself, and when checking for logs on a medium sized mailserver (around 10K mails each day) you see those everyday
1) Connection timed out with domain.tld: this means that there's something wrong on that server. Either it's down, the service is down, refusing connections, not updated MX for a domain, etc
2) Host unknown: if someone in your domain sends a mail to firstname.lastname@example.org, there's a big chance that these domain (or subdomain) doesn't exist. Or if it exists, it doesn't have a mail server. Most of this errors are just mistyped mail addresses or old mail addresses from old contact lists.
You'll see MANY errors, in quantity & variety while checking a mailserver log. Don't panic, the fact that you work hard to keep your mailserver working & functional, doesn't mean that the people managing x.y.z mailserver puts the same hard work on their servers.