> On 2 Sep 2025, at 08:26, Luke Thompson via mailop <mailop@mailop.org> wrote: > > G'day, > > We are seeing this error across some servers/domains. > > SMTP Error 451 4.7.500 Server busy. Please try again later. > This seems to relate to Microsoft determining a change in sending behaviour. > > The error occurs when the recipient mail server's Exchange Online > Protection (EOP) service detects a significant change in the sending > mail server's previously established sending patterns. > Checking the sender.office.com system, it reports the most problematic client > machine as fine: > > The IP address in question is not currently blocked in our system. > So, given it is causing a bit of havoc and vendors like cPanel have just > issued an article 4 days ago about it (so incidence rate is up), what's the > go? > > https://support.cpanel.net/hc/en-us/articles/4408492985111-SMTP-Error-451-4-7-500-Server-busy-Please-try-again-later > > If we can't resolve anything and it's deferring it many times (5+) then what > can we really do? >
Have you looked to see if there were changes in your sending pattern recently that would trigger Microsoft to temp fail mail from you? Could it be that there are traffic patterns that look like maybe one of your customers is compromised or otherwise sending mail that is problematic? Temp failing (451 and the like) is a common reaction that is not blocking (that would be an actual block) to traffic that seems suspicious. Is it all the traffic from your system or just some of the traffic? When in the SMTP transaction is the block happening? At connection? After DATA? Elsewhere? That may lead you to some information about what is triggering the issue. Have you tried pausing deliveries for a period of time (1 - 2 hours or longer)? And then slowly restart sending? laura -- The Delivery Expert Laura Atkins Word to the Wise la...@wordtothewise.com Delivery hints and commentary: http://www.wordtothewise.com/blog
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