Snort alerts of the ICMP relationship with smtp connection?

Hi,

I am using Snort version Version 2.3.2 (Build 12).
I have in my snort logs the alerts:

366 - ICMP Ping *nix
384 - ICMP Ping
368 - Ping BSDtype

I investigated my others systems logs and in the time
that this alert is recorded is the same that
registered smtp connection in the maillog arquive from
my postfix server.

The source IP address in snort's log is equal the
destination IP address in the maillog to smtp
connection.

My smtp server is a Postfix version 1.1.3.

This alerts can to be generated by destination mail server when
it receives mails?

Any friends have said about the path MTU discovery, it's true? How can i to confirm this?

This alerts is a false positive?

Thanks by help