strange traffic on UDP port 53

[Ronald Belchez <meukone () yahoo co uk>]

Hi All,
We don't have a firewall and is just relying on Access-list on our border 
router. After i applied the new access-list I am continously receiving 
the logs showed below. The destination IP is our mail server (not running 
any DNS service) while the source IP (unsolicited and using source port 
with some sort of incremental patterm, the denied packets logs is also 
continuous now for about 4 days) I am not aware of any trojan or worm 
using the below. I already tried searching google but cannot find the 
explanation or something that might help me understand the below.... 
Please advise.

--logs starts here---
denied udp XX7.Y3.71.242(54067) -> XX3.Y1.246.66(53), 1 packet
denied udp XX7.Y3.71.242(54070) -> XX3.Y1.246.66(53), 1 packet
denied udp XX7.Y3.71.242(53967) -> XX3.Y1.246.66(53), 2 packets
denied udp XX7.Y3.71.242(53972) -> XX3.Y1.246.66(53), 2 packets
denied udp XX7.Y3.71.242(53979) -> XX3.Y1.246.66(53), 2 packets
denied udp XX7.Y3.71.242(53989) -> XX3.Y1.246.66(53), 2 packets
denied udp XX7.Y3.71.242(54003) -> XX3.Y1.246.66(53), 2 packets
denied udp XX7.Y3.71.242(53982) -> XX3.Y1.246.66(53), 34 packets
denied udp XX7.Y3.71.242(54009) -> XX3.Y1.246.66(53), 2 packets
denied udp XX7.Y3.71.242(54027) -> XX3.Y1.246.66(53), 2 packets
denied udp XX7.Y3.71.242(54035) -> XX3.Y1.246.66(53), 2 packets
denied udp XX7.Y3.71.242(54042) -> XX3.Y1.246.66(53), 2 packets

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