SMTP Response Times Spiking

About the middle of the month I started to get SMTP service down messages from a service monitor site I had signed up with. Initially I wasn't concerned but I started to get at least one notice a day. I decided to monitor the server a bit more closely so I setup a virtual appliance on my home server to monitor SMTP response times a bit more frequently -- I let the monitoring application run for a few days to gather data and noticed an unusual trend.

SMTP response times would spike dramatically during business hours -- up to almost 19 seconds at some times. However after business hours and on the weekends response times would return to normal.

My mail volume is really low for my VPS so I don't think it is the problem -- I even ran a mail stress test and pushed close to 500 messages through in 30 minutes (more than I normally get in a day) and I didn't see any SMTP response time spikes. I have also set up monitors to watch some other sites I have running on shared servers with other providers and do not see any unusual SMTP response issues with them at all.

system information is below:

Linux OS: CENTOS
Kernel Version: 2.6.18-028stab053.4
Hardware Information: VPS (1500MB RAM)
Exim Version: Exim version 4.69 #1 built 16-Mar-2009 16:41:00
Spamd Version: SpamAssassin Server version 3.2.4
running on Perl 5.8.8

Control Panel: CPANEL/WHM

TOP:
------------------------------
top - 13:26:06 up 2 days, 19:23, 1 user, load average: 0.00, 0.00, 0.00
Tasks: 54 total, 2 running, 52 sleeping, 0 stopped, 0 zombie
Cpu(s): 0.0%us, 0.0%sy, 0.0%ni,100.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 1572864k total, 206196k used, 1366668k free, 0k buffers
Swap: 0k total, 0k used, 0k free, 0k cached

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
1 root 15 0 1984 648 560 S 0 0.0 0:02.76 init
1590 nobody 16 0 61400 22m 10m S 0 1.5 0:07.71 httpd
1992 root 18 0 35112 31m 2196 S 0 2.1 0:15.58 spamd
7196 root 34 19 3744 1768 840 S 0 0.1 0:00.15 cpanellogd
7256 root 18 0 8464 5964 1772 S 0 0.4 0:31.80 tailwatchd
7292 root 18 0 5800 3776 936 S 0 0.2 0:00.00 cphulkd.pl
7321 root 23 0 14684 7580 440 S 0 0.5 0:00.00 cpdavd
7427 root 15 0 15152 12m 1492 S 0 0.8 0:15.25 lfd
7491 nobody 15 0 62004 23m 11m S 0 1.5 0:04.87 httpd
7599 root 15 0 27388 24m 2168 S 0 1.6 0:05.53 spamd
7661 nobody 15 0 61776 23m 11m S 0 1.5 0:03.90 httpd
7907 nobody 18 0 61724 22m 10m S 0 1.5 0:04.60 httpd
9710 root 18 0 3212 1116 576 S 0 0.1 0:00.70 crond
12179 root 15 0 18348 8036 988 S 0 0.5 0:00.67 cpsrvd-ssl
13727 root 15 0 9924 2848 2296 R 0 0.2 0:00.03 sshd
16016 nobody 15 0 60652 21m 10m S 0 1.4 0:05.26 httpd
16017 nobody 15 0 61984 22m 10m S 0 1.4 0:07.89 httpd

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vmstat 5 5
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procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu------
r b swpd free buff cache si so bi bo in cs us sy id wa st
0 0 0 1367104 0 0 0 0 29 6 0 21 0 0 100 0 0
0 0 0 1366616 0 0 0 0 0 0 0 19066 0 0 100 0 0
0 0 0 1366284 0 0 0 0 0 0 0 25372 0 0 100 0 0
0 0 0 1366868 0 0 0 0 4 3 0 19395 0 0 100 0 0
0 0 0 1366904 0 0 0 0 0 0 0 19464 0 0 100 0 0

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