Errors like this are shown on high usage servers and ports so it is common to see it on http and even imap ports:
possible SYN flooding on port 80. Sending cookies.
The Linux kernel will even detect flooding on OpenVZ containers:
possible SYN flooding on ctid 6000, port 993. Sending cookies.
In many cases this is not an issue and is more so simply a result of regular, but high usage traffic.