SYN Flood

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If port 25 connection on your server are only intermittently available you may be under attack by a SYN flood.

When it happens, does the command:

   netstat -tuna | grep :25 | grep SYN_RECV

show a lot of connections?

If so, it's a "SYN flood attack" - see the page at:

   http://linoxide.com/firewall/snapshot-syn-flood-attack/ 

for details.