Yes, we`ve been trying to find out what`s going on with some mails too.. Many get stuck in the spools with the "Receiving status" and never get delivered. Apparently the larger they are, the more likely to get stuck.
We`ve noticed that if we move just the .eml file (without the corresponding .hdr), then the mail might get delivered.
Also, after watching the logs intensively, these things come up:
[2017.11.27] 16:32:28 [IP][8846677] rsp: 220 ServerName | Mon, 27 Nov 2017 16:32:28 +02:00 GMT | Mon, 27 Nov 2017 14:32:28 +0000 UTC | Attention IP - Sending unsolicited commercial or bulk e-mail to this computer network is prohibited
[2017.11.27] 16:32:28 [IP][8846677] connected at 11/27/2017 4:32:28 PM
[2017.11.27] 16:32:28 [IP][8846677] cmd: EHLO SendingServer
[2017.11.27] 16:32:28 [IP][8846677] rsp: 250-ServerName Hello [IP]250-SIZE 47185920250-AUTH LOGIN CRAM-MD5250-STARTTLS250-8BITMIME250 OK
[2017.11.27] 16:32:28 [IP][8846677] cmd: STARTTLS
[2017.11.27] 16:32:28 [IP][8846677] rsp: 220 Start TLS negotiation
[2017.11.27] 16:32:28 [IP][8846677] cmd: EHLO SendingServer
[2017.11.27] 16:32:28 [IP][8846677] rsp: 250-ServerName Hello [IP]250-SIZE 47185920250-AUTH LOGIN CRAM-MD5250-8BITMIME250 OK
[2017.11.27] 16:32:28 [IP][8846677] cmd: MAIL FROM:<Sender> SIZE=21565874 AUTH=<>
[2017.11.27] 16:32:28 [IP][8846677] rsp: 250 OK <Sender> Sender ok
[2017.11.27] 16:32:29 [IP][8846677] cmd: RCPT TO:<Receiver>
[2017.11.27] 16:32:29 [IP][8846677] rsp: 250 OK <Receiver> Recipient ok
[2017.11.27] 16:32:29 [IP][8846677] cmd: DATA
[2017.11.27] 16:32:29 [IP][8846677] rsp: 354 Start mail input; end with <CRLF>.<CRLF>
[2017.11.27] 16:32:30 [IP][8846677] rsp: 250 OK <ionel-aurelian rau <Sender>> Sender ok
[2017.11.27] 16:32:30 [IP][8846677] cmd: 5mw
[2017.11.27] 16:32:30 [IP][8846677] rsp: 500 command unrecognized
[2017.11.27] 16:32:30 [IP][8846677] cmd: fagC8ZYZRKZ8ojhdotY+lpupGajv3Bh9MaBPnJ8bZgmmVw3JoWxjubR8b1cNswte3ZnTiu++j6ZM
[2017.11.27] 16:32:30 [IP][8846677] rsp: 500 command unrecognized
[2017.11.27] 16:32:30 [IP][8846677] cmd: 7KYbAmVrPEd9vrfX6Wgz50Nl+6PhPqY/+Pyt9D3tP65Emg99QrZ2f22o7yoEhddrs/zdl6p/v1yR
[2017.11.27] 16:32:30 [IP][8846677] rsp: 500 command unrecognized
[2017.11.27] 16:32:30 [IP][8846677] cmd: tH85fDtWJJenzSvs0H8VR//3V1L6LfJRQv3vr8D7inz1deRnN+QZOhV+qZhq/EXTLyb6ehsSzjcs
[2017.11.27] 16:32:30 [IP][8846677] rsp: 500 command unrecognized
[2017.11.27] 16:32:30 [IP][8846677] cmd: 7JBd0x3qexWqJSlfn6J8Xory4hTlw2i51Xjdm2S85hSfJBrV1IVb76WPSxvx+Vapb4G3AGg3hXZp
[2017.11.27] 16:32:30 [IP][8846677] rsp: 500 command unrecognized
[2017.11.27] 16:32:30 [IP][8846677] cmd: wyOLB1HPLYCu09eePLzlcRnLhkv7W7uI92ru3fkO8/borxw9kWoF0RMgegGWdZhjs/EWS4G2u6Th
[2017.11.27] 16:32:30 [IP][8846677] rsp: 500 command unrecognized
[2017.11.27] 16:32:30 [IP][8846677] cmd: 4WFaOlEenwvVQtuwT83tHMT3f+UKbNPIs94Hl9uhx6I/f3aP8bnuNYsdsGZp7/25eAGJ+3fCPWz/
[2017.11.27] 16:32:30 [IP][8846677] Closing transmission channel: too many bad commands
[2017.11.27] 16:32:30 [IP][8846677] rsp: 421 Too many bad commands, closing transmission channel
[2017.11.27] 16:32:30 [IP][8846677] disconnected at 11/27/2017 4:32:30 PM
Apparently, with larger mails, especially those with attachements, a lot of "500 command unrecognized" occur during the SMTP session followed after "421 Too many bad commands". This never occurred before the update to Version 16.3.6535.
PLEASE HELP AS MAILS ARE NOT BEING PROPERLY DELIVERED!!!