email hacking attempt?

[David Fletcher]

I keep getting from time to time rejection entries in my postfix logs
of the following kind:-

Feb 28 13:47:05 ServerIV postfix/smtpd[37050]: NOQUEUE: reject: RCPT
from sender48-eu-west-1-p.emailage.com[52.212.158.48]: 550 5.1.1 <
5ea11ebd64854dfbbe03377e at thefletchers.net>: Recipient address rejected:
User unknown in local recipient table; from=<smtp at emailage.com> to=<
5ea11ebd64854dfbbe03377e at thefletchers.net> proto=SMTP
helo=<emailage.com>

This is obviously never going to be a valid email address so I wonder
what is going on here? Are these attempting to exploit some known
buffer overflow or other flaw in a mail server?

Just curious so I thought I'd ask as the list seems to be a little more
quiet today.

Dave