Network monitoring software (free if possible)

I want to know if there is software that can monitor network problems

Situation #1
Recently my web server had an issue where we were not receiving any e-mail.but we could send so this is why this went un-noticed for so long
no one really noticed until hours later.    So after looking at what was wrong, we could not contact the mail server from the uoutside.  No big deal, reboot and problem fixed. But I would have loved to have known much sooner.

Is there a software that can monitor from the outside and check specific ports or ip addresses like my and let me know if there are problems?

Situation #2
Everynow and then my file server will not go down but rather you can't get files you can navigate to them, but when you try and open them they are unavailable or can't be opened like the server is down.

I have LAN monitoring software but since the server is actually up and running and I can log into it its technically not down.
This is not such a big deal since so many people use this server I know when its not working right within minutes.  But I would still like to know before suoervisors start calling me freaking out.

Any suggestions are greatly appreciated.
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Situation #1
IsItUp from Tarosoft.

Situation #2
A good file server doesn't lock up like that. You might want to do a check up / maintaining on your file server. Perhaps it need to be updated. If the problem keep re-occuring, and you are unable to find a permanent solution for it, you can always reinstall that server (of course, backup all stuffs first, and do it during the weekend).

Hope this help,
Take a look at these also:
Servers Alive:
Level Platforms: [BLINK:] br />com/

I use Servers Alive for non-contract customers because it provides network, service, ports, etc... monitoring for less than $300.  It can also send an email to an outside IP address and then check your mail server to see if it received the email.  You do have to dedicate 1 mailbox for this function that you do not touch and that is not part of any other distribution groups.  But it works great.  An ISP I worked for in the past loved this as they could test each of their SMTP, POP, and exchange servers and know of a problem before customers could notice the issue.  

For contract customers I use Level Platforms as they do cost more.  I do like some of the feature of Kaseya.   I haven't tried hounddog but they must be good because our Level Platforms rep almost freaked when he saw the hounddog web page up on my pc.  I do that often.  When a vendor comes it to negotitate a price I just happen to have their competitors site up on my pc.  It tends to make them a lot more flexible on the charges.
For webserver, mailserver and other services  monitoring , you can use nagois.
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Kamran ArshadIT AssociateCommented:

You need a service monitor not a Network monitor. Below is a good list of service monitors;

BigBrother                                 For *NIX based machines
BigSister                                 For *NIX based machines
Hobbit                 For *NIX based machines
Hound-Dog         Propriety
Level Platform                 Propriety
Kaseya                                 Propriety
N-Able                                 Propriety
codeyelloAuthor Commented:
I have been looking at many of these.
Anyone used a free service monitor to monitor the mail server?
I was wondering if I could use a website monitor to monitor the ip address and or mail server domain name, from the outside

Trying to get my company to buy something for IT is like pulling teeth so I try the free approach first.  
have you read documentation for nagios ??
read the documentaion for nagois, it has the ablicity to monitor mail server.
codeyelloAuthor Commented:
"have you read documentation for nagios ??
read the documentaion for nagois, it has the ablicity to monitor mail server."

it said it was for linux.  I run a pure Ms network

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Hi check this one

Nagios for monitoring windows mecine
codeyelloAuthor Commented:
Thank you I really wanted to try this program and was bummed when it didn't work on windows
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