Hum. No response here I see.

We're now seeing same.

[2021.01.27] 14:52:39.060 [15675602] CMD: RCPT TO:<XXXXXXX> NOTIFY=FAILURE
[2021.01.27] 14:52:39.341 [15675602] RSP: 250 Recipient <XXXXXXX> Ok
[2021.01.27] 14:52:39.341 [15675602] CMD: DATA
[2021.01.27] 14:52:39.622 [15675602] RSP: 354 Ok Send data ending with <CRLF>.<CRLF>
[2021.01.27] 14:52:42.904 [15675602] RSP: 250 1105714 bytes received: 1794606142 (8d4d8c979ffc4ca29c2020bc7e635630-20210128)
[2021.01.27] 14:52:42.904 [15675602] CMD: QUIT
[2021.01.27] 14:52:43.185 [15675602] RSP: 221 ESMTP closing connection
[2021.01.27] 14:52:43.185 [15675602] Attempt to ip, '[REMOTE IP]' success: 'True'
[2021.01.27] 14:52:43.185 [15675602] Delivery for YYYYYYY to XXXXXXX  has completed (Delivered)

So it shows the Delivery has completed (Delivered)

But... we see NOTIFY=FAILURE 

* Sender has no bounce email

* Recipient has no incoming message.

* SMTP logs so no record of this email.

Lost in space.

We'll open a support ticket. but Bruce did you ever solve this?

NOTIFY=FAILURE on RCPT lines for outgoing malis, tells the remote server that you want a DSN when a delivery failure for this recipient happens (which is a common default when it is not explicitly specified)
https://tools.ietf.org/html/rfc3461#section-4.1

This line in your log shows that the remote server accepted the mail.

RSP: 250 1105714 bytes received: 1794606142 (8d4d8c979ffc4ca29c2020bc7e635630-20210128)  

At this point it's up to the remote server to deliver the mail in the recipient mailbox OR return a DSN to the sender if it failed.

Has the recipient checked his Spam folder ? It was maybe delivered directly to the user Spam folder.

If that's not the case, your recipient should check with admin of the remote server where the mail is gone as it has been accepted by their server.

Hmm, when you say SMTP logs has no record of this e-mail, you mean the SMTP log of the sending server (SmarterMail?). 

It's a bit confusing but the "SMTP log" on SmarterMail is only for incoming SMTP connections to the server. 

The SMTP transactions for mails sent to the world by the server are logged in the "Delivery log" which is exactly what you pasted here :)

Hope this helps. 

Gotcha

Yeah, SMTP log show inbound connexions (from external mail servers and from customers that connect your server with smtp) So you could consider these as outbound aswell. But in reality this is inbound to the server from your customer PC.

Then when SmarterMail process to send the mail to a remote host, this is in delivery log.

A bit confusing and hard to explain. Hope you get it :)