We have been receiving a number of reports regarding bounce backs and delayed delivery reports.
Upon looking into the attempted recipient domains we found that a commonality was that they were using Office 365 for their email solution.
After reviewing the logs, we are seeing a pattern of timeouts and connection rejections due to meeting a threshold of connections open to O365 servers.
Log:
[2016.06.30] 12:24:03 [25173] Delivery started for client1@domainA.com at 12:24:03 PM
[2016.06.30] 12:24:15 [25173] Skipping spam checks: User authenticated
[2016.06.30] 12:24:18 [25173] Starting local delivery to client2@domainA.com
[2016.06.30] 12:24:18 [25173] Delivery for client1@domainA.com to client2@domainA.com has completed (Delivered) Filter: None
[2016.06.30] 12:24:18 [25173] End delivery to client2@domainA.com
[2016.06.30] 12:24:18 [25173] Sending remote mail for client1@domainA.com
[2016.06.30] 12:24:18 [25173] Initiating connection to 207.46.163.215
[2016.06.30] 12:24:18 [25173] Connecting to 207.46.163.215:25 (Id: 1)
[2016.06.30] 12:24:18 [25173] Binding to local IP 192.168.100.40:0 (Id: 1)
[2016.06.30] 12:24:18 [25173] Connection to 207.46.163.215:25 from 192.168.100.40:52466 succeeded (Id: 1)
[2016.06.30] 12:26:18 [25173] The smtp session has timed out.
[2016.06.30] 12:36:19 [25173] Sending remote mail for client1@domainA.com
[2016.06.30] 12:36:19 [25173] Initiating connection to 207.46.163.247
[2016.06.30] 12:36:19 [25173] Connecting to 207.46.163.247:25 (Id: 1)
[2016.06.30] 12:36:19 [25173] Binding to local IP 192.168.100.40:0 (Id: 1)
[2016.06.30] 12:36:19 [25173] Connection to 207.46.163.247:25 from 192.168.100.40:52657 succeeded (Id: 1)
[2016.06.30] 12:38:19 [25173] The smtp session has timed out.
[2016.06.30] 12:38:19 [25173] Delay notification email written to 236425276.eml
[2016.06.30] 12:48:22 [25173] Sending remote mail for client1@domainA.com
[2016.06.30] 12:48:22 [25173] Initiating connection to 207.46.163.247
[2016.06.30] 12:48:22 [25173] Connecting to 207.46.163.247:25 (Id: 1)
[2016.06.30] 12:48:22 [25173] Binding to local IP 192.168.100.40:0 (Id: 1)
[2016.06.30] 12:48:22 [25173] Connection to 207.46.163.247:25 from 192.168.100.40:52866 succeeded (Id: 1)
[2016.06.30] 12:50:22 [25173] The smtp session has timed out.
[2016.06.30] 13:00:23 [25173] Sending remote mail for client1@domainA.com
[2016.06.30] 13:00:24 [25173] Initiating connection to 207.46.163.247
[2016.06.30] 13:00:24 [25173] Connecting to 207.46.163.247:25 (Id: 1)
[2016.06.30] 13:00:24 [25173] Binding to local IP 192.168.100.40:0 (Id: 1)
[2016.06.30] 13:00:24 [25173] Connection to 207.46.163.247:25 from 192.168.100.40:53076 succeeded (Id: 1)
[2016.06.30] 13:02:24 [25173] The smtp session has timed out.
[2016.06.30] 13:02:24 [25173] Initiating connection to 207.46.163.170
[2016.06.30] 13:02:24 [25173] Connecting to 207.46.163.170:25 (Id: 2)
[2016.06.30] 13:02:24 [25173] Binding to local IP 192.168.100.40:0 (Id: 2)
[2016.06.30] 13:02:24 [25173] Connection to 207.46.163.170:25 from 192.168.100.40:53101 succeeded (Id: 2)
[2016.06.30] 13:04:24 [25173] The smtp session has timed out.
[2016.06.30] 13:04:24 [25173] Initiating connection to 207.46.163.138
[2016.06.30] 13:04:24 [25173] Connecting to 207.46.163.138:25 (Id: 3)
[2016.06.30] 13:04:24 [25173] Binding to local IP 192.168.100.40:0 (Id: 3)
[2016.06.30] 13:04:24 [25173] Connection to 207.46.163.138:25 from 192.168.100.40:53147 succeeded (Id: 3)
[2016.06.30] 13:04:24 [25173] RSP: 421 4.3.2 The maximum number of concurrent server connections has exceeded a per-source limit, closing transmission channel (BN1AFFO11FD054.protection.gbl)
[2016.06.30] 13:04:24 [25173] CMD: QUIT
[2016.06.30] 13:34:32 [25173] Sending remote mail for client1@domainA.com
[2016.06.30] 13:34:32 [25173] Initiating connection to 207.46.163.247
[2016.06.30] 13:34:32 [25173] Connecting to 207.46.163.247:25 (Id: 1)
[2016.06.30] 13:34:32 [25173] Binding to local IP 192.168.100.40:0 (Id: 1)
[2016.06.30] 13:34:32 [25173] Connection to 207.46.163.247:25 from 192.168.100.40:53685 succeeded (Id: 1)
[2016.06.30] 13:36:32 [25173] The smtp session has timed out.
[2016.06.30] 13:36:32 [25173] Bounce email written to 236425790.eml
[2016.06.30] 13:36:32 [25173] Delivery for client1@domainA.com to abc@domain.com has completed (Bounced)
[2016.06.30] 13:36:32 [25173] Delivery finished for client1@domainA.com at 1:36:32 PM [id:236425173]
NDRs:
"Subject: Delivery delay notification
The server(s) that the message "RE: Email Subject XYZ" is attempting to be
sent to has temporarily delayed the delivery for the following recipient(s):
abc@domain.com
There will be up to 2 more delivery attempt(s) of this message. Do not
re-send your message until there are no more delivery attempt(s) and the
messages bounces back to you."
_____________________________________________________________
"Subject: Delivery Failure
Could not deliver message to the following recipient(s):
Failed Recipient: abc@domain.com
Reason: Remote host said: 601 Attempted to send the message to the following
ip's:
207.46.163.247"
Has anyone encountered this before? Any solutions or thoughts?