Mail spooled but never delivered

--_000_DBAP195MB09563B4EBA97A83FEDADEBDAC399ADBAP195MB0956EURP_
Content-Type: text/plain; charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable

Hi all,

I'm running James 3.8.2 (Guice-JPA ) on Windows Server 2019 and I have a se=
tup where James acts as an outbound relay for authorized addresses.
Most of the time this works well, however, in small number of cases (~2%) J=
ames will accept and spool the mail but never deliver it.
The usual pattern in the log is like this:

INFO 14:27:10 [smtpserver-global-9] org.apache.james.smtpserver.SendMailHan=
dler - Successfully spooled mail Mail1745843230572-66ed5e2c-dcf7-461b-bba6-=
113143787e10 with messageId [email protected]=
main.com from [email protected] on sendinghost.mydomain.com /x.x.x.x for [yy@=
gmail.com]
...
INFO 14:27:11 [RemoteDelivery-Process-38] org.apache.james.transport.mailet=
s.remote.delivery.MailDelivrerToHost - Mail (Mail1745843230572-66ed5e2c-dcf=
7-461b-bba6-113143787e10-to-gmail.com) with messageId 1194519856.39668.1745=
[email protected] sent successfully to gmail-smtp-in.l.goo=
gle.com. at 142.250.147.27 from [email protected] for [[email protected]]

But sometimes I will only see the first entry (e.g. the mail is spooled but=
 never delivered).
I have checked the spool and error queues and both are empty,
There is nothing in the log indicating an error.
It is like the mail has just vanished.
The error does not seem to correlate with sender or the recipient domains.

Any ideas on how to troubleshoot further would be greatly appreciated.

Best regards,

Soeren

--_000_DBAP195MB09563B4EBA97A83FEDADEBDAC399ADBAP195MB0956EURP_--