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.