If you are living the Exchange Online Hybrid dream like so many, you often run into oddities around message flow. One encountered recently was a nagging NDR an Exchange Online user would get when sending to an old-school Distribution List.
This message wasn’t delivered to anyone because there are too many recipients. The limit is 100. This message has 198 recipients.
X-Supplementary-Info: < #5.5.3 smtp;550 5.5.3 RESOLVER.ADR.RecipLimit; too
Now the first thought is some Exchange Online limitation, but the NDR was actually generated by an on-premises server and, as we all know, DLs count as one recipient right? Besides, I wasn’t aware of any limit anywhere in Exchange of 100 – except the pickup directory for the transport service.
Regardless, due diligence required some checking and the sender ( who was part of the list) had the standard recipient limits in place:
And if you didn’t already know, 500 is the default in Exchange Online and that can not be modified.
The other members of the group were all on-prem, so I checked their recipient limits and, as expected, those were set to the default of “unlimited”.
Hmm, Did someone mess with the on-prem org limits? Didn’t make sense since this was the only user reporting this. But being the good scout, I checked anyway and that value was set much higher than 100.
So where the heck was that coming from?
Checked receive connector limits, looked through SMTP protocol logs. No smoking gun.
Then it occurred to me, was it possible that the remote mailbox for the 365 sender was the culprit?
Sure enough, there it was:
Set-Remotemailbox doesn’t let you change that attribute, so I cleared it with ADUC and the birds sang once again.
Reconstructing this, I can only assume that the on-prem mailbox had this value set for some reason and during the move to Exchange Online, it was preserved for the remote mailbox.
The interesting piece to remember is that the DL was expanded in Exchange Online and because majority of the list members were still on-prem, the recipient limits came into play on the on-prem object.