Now to the 'accidental' solution for my particular setup. I noticed by chance that a new folder called 'Bulk Mail' has been created in my Outlook profile - how that got created (Outlook, my own spam filtering (Norton), etc.) I've no idea. Googling it there seems to be conflicting views as to whether it is an Outlook system folder or not, but it does look as though it could be deleted which seems to suggest it is not. I have always used Outlook and POP email for retrieving my btinternet-addressed email, and this Bulk Mail folder is new with my Office 365 setup. Anyway, I looked in that Bulk Mail folder and lo and behold, there are all my BT Yahoo mail spam emails. As all mail going to my BT Yahoo Mail Inbox is now autoforwarded and delivered immediately to my Outlook Inbox, I can only assume the spam emails are being retrieved separately by the POP retrieval process. Following a little more inconclusive Googling the only other clue I have is that if you look into the details of them, the spam email headers all seem to contain the string 'X-YahooFilteredBulk' whereas the non-spam mail headers do not. Purely by chance this works even better for me than what I was trying to achieve in the first place! I suppose Outlook must in some way use that or other data in the header detail to route the mail to the Bulk Mail folder. I post this lengthy ramble here as it may be of some small interest or help to others who are trying to resolve a similar problem.
Earlier versions of BTYahoo webmail gave the option of preceding POP forwarded spam mails with 'Bulk' rather than 'Spam'. The option doesn't appear to be available now and prefixes with the word 'Spam' rather than 'Bulk'. However, as you have discovered, the header still contains 'X-YahooFilteredBulk' which is what I use to filter on in Outlook 2010.
"I think probably because the autoforwarding marks the mail as 'read' and the POP retrieval which occurs later thinks it has already seen those mails."
POP retrieval normally relies on a table in the email client to decide which emails have already been downloaded. I use POP on my laptop to see if there are any urgent emails, then I use POP from my desktop to download all those emails and archive the ones I need to keep. I tried IMAP, but this synchronises the emails between the computers and the servers. When I delete an email in the laptop is disappears so the desktop does not get the email.
"Now to the 'accidental' solution for my particular setup. I noticed by chance that a new folder called 'Bulk Mail' has been created in my Outlook profile - how that got created (Outlook, my own spam filtering (Norton), etc.) I've no idea."
This sounds typical of an IMAP connection, I don't know about other email services. With IMAP, the server synchornises folders with the computer(s). 'Bulk Mail' is what some email services call the folder where they dump spam emails.
I think you will find that the posters on this thread know the difference between IMAP and POP.
As regards the bulk folder, you are correct in suggesting that is typical of an IMAP connection and that is where the email client would normally dump spam emails however in this instance the OP was clear that he is using POP.
Yes, definitely POP not IMAP. Moreover I don't have any folder in my BT Yahoo mail account called 'Bulk' or 'Bulk Mail'. In fact IMAP can't be used with 'extended' (legacy) addresses with Exchange Server I suppose because since all folders in the legacy accounts would be synchronised, and since a user may have several legacy email accounts, that may potentially lead to duplicate folder names.
I still haven't discovered what created the Bulk Mail folder in Outlook. I looked back at the Outlook PST file just prior to moving to exchange server and it didn't exist then.
I think the "bulk" folder is created by the email client. It is not a spam folder as such, rather a folder that is used for email that has more than one recipient in the address fields which just so happens is the case with junk email. You may find that if you are on a genuine mailing list any emails from that list will end up there as well.
That's what I think too, but when I was using Outlook (with POP) to retrieve my BT Yahoo email prior to migrating to Exchange Server there was no Bulk Mail folder created. Having checked as best I can, it seems to contain only BT Yahoo spam email dating back to the date when I first set Exchange Server up.
In my BT Yahoo mail settings, the option that is supposed to be there to include spam emails when setting up POP mail isn't available for editing on my web interface - see screenshot.
I thought this was because I have autoforwarding turned on (those options appear to be mutually exclusive), but when I disabled that, whilst the POP option became editable again, the option to include spam as shown in various Help pages did not appear. Also, note that in the POP description in the screenshot it is specifically stated that spam will NOT be downloaded - despite that it definitely is being downloaded and put in the Bulk Mail folder in Outlook. I have checked in Outlook and I can't find any rules that have been set up.
I'm on many mailing lists, but no other bulk mail appears to be being routed to the Bulk Mail folder.
Probably of little help but I use Outlook 2010 POP3 with BTYahoo mail set to download spam previously annotated 'Bulk' but now annotated 'Spam'. I don't have a bulk folder in Outlook, I just have a folder that I created called 'Junk' that receives mail via an Outlook rule that checks for 'X-YahooFilteredBulk' in the header. It sounds like an Exchange Server thing.