Nikolaos Milas:
> On 8/6/2022 5:44 ?.?., Wietse Venema wrote:
> > Possible causes (there may be more):
> >
> > - There is a problem with the network connection between mailgw1
> > and mailgw1 that causes some connections to have excessive retries.
> > This could be a data-dependent problem. About 20 years ago, someone
> > fixed a Postfix networking problem by replacing bad network hardware
> > (a different port on a switch).
> >
> > - There is a problem in the file system that causes delays in the
> > fsync() system call. Postfix will not reply that the mesasage has
> > been "received" before that system call completes successfully. If
> > you are using a networked file system, see my previous point. fsync()
> > performance also depends on how a disk drive manages its cache.
> >
> > - vmail2 is using header_checks, body_checks, or smtpd_milters that
> > take an insane amount of time. These are by their nature data-dependent.
> > Sorry, no time to examine up your Postfix configuration.
>
> Thank you Wietse for this analysis.
>
> My big question is why this happens *ONLY* to particular messages, esp.
> those originating from wetransfer.com and sharepointonline.com, and it
> happens *consistently* to those.
Did I say data-dependent? I thought so.
> These messages (with delays) are a *very* small percentage of the total
> mail we receive.
>
> Moreover, we don't do any header / body? etc checks. All such
> checks
I could swear that vmail has header or body checks, along with
smtpd_milters.
> deliver mail to the final recipient.
>
> As I wrote to Victor, the only milter is the DKIM signing one, used for
> outgoing messages.
I agree that DKIM is unlikely to use upo 255 seconds even if
you had turned on signature checks by mistake.
> How could we further investigate the issue, e.g. by more thoroughly
> (i.e. in high detail) logging postifx activity on mails from those domains?
Betetr network monitoring. Something is taking 255 seconds to time out.
Wietse
