Strange incoming probes from PlusNet addresses (Mail servers perhaps ??)

[#1]

Not really sure this is the right forum for this but let's put it here for starters...

I've just had cause to look at our office Firewall log ( for a completely different reason) and noticed there are bursts of incoming TCP packets from various addresses in the 84.83.229 & 84.93.238 subnets. These lookup as mailc<nn>.plus.net (are they the Cloudmark servers by any chance ?)

For information,  the office connection is NOT a PlusNet one ( it's Zen ) and it's not running a mail server.

Would someone ( @bobpullen perhaps ? ) care to explain where these incoming requests are coming from and why ?

I can PM an extract from the firewall log if it's useful

[#2]

They're the mail collection servers. So it's likely in response to something on the network polling mail.plus.net or imap.plus.net.

[#3]

Thanks for the reply Bob. Yes, my phone would have been on the network polling the imap server for my PlusNet account, but surely that would be a solicited response and therefore be allowed through the firewall via an open NAT session ?

These seem to be unsolicted requests and are being blocked by the Firewall.

edit: I wonder if it's timing out on a reply from the imap server and the NAT pinhole is closed before the reply comes back ?

[#4]

There's an old thread knocking about where this was discussed. On mobile at the mo so will have a search for it when it's more convenient.
Don't think it ever came to a worthwhile conclusion, but it was a valid question at the time.

[#5]

Thanks Bob. No great rush, I'm working from home tomorrow , so won't be back in the office till Monday.

[#6]

@bobpullen I think this https://community.plus.net/t5/Email/IMAP-14300/td-p/1339262 is the old thread you were referring to.

Yup, looks like it's exactly the same issue that I am seeing , ACK's on port 14300 from the mail servers.hwall

As the OP in that thread @Simon we use Smoothwall as well.

Might be better if we return to  that old thread and leave this one...