This is a bit of a weird one and I will try to explain as best I can.
I have a Sharepoint 2010 intranet site. It has a number of workflows set up using Nintex Workflow, some of which send various email notifications to users within our network (AD/Exchange).
Over the past few days, a number of these emails have also arrived in the wrong mailboxes as well as the correct one. The mailboxes in which they are duplicated are seemingly random (not Admin or special groups), though they may be the mailboxes of recent actual recipients of workflow emails. (i.e. the previous recipient of an email from one workflow may be the 'copied' recipient in a subsequent workflow - unconfirmed though).
This problem does not seem to be related to a specific workflow nor to specific users. There have been no changes to the workflows recently nor the Sharepoint configuration.
My suspicion is that the problem may be related to Exchange but it only seems to be affecting emails that come from the Sharepoint workflows. These are all sent from a single mail address.
The emails that arrive in the correct mailbox and the wrong one are identical. Addressed to the correct person. No mention of the wrong mailbox in the headers.
We did recently update some firewall policies (TMG) but I'm not sure how this would be related.
Has anyone experienced anything similar to this? I am not sure what additional information I can provide at this stage, but please ask and I will update the question as needed.