I’ve been using SpamSieve for about 18 years. I have never really solved one mystery. I’m not sure if it’s a SpamSieve thing or a macOS Mail thing or a combination of both, but I hope you can clarify or even solve it.
On my Mac Studio, using Mail, SpamSieve is installed. It puts spam in the folder that is in the inbox > Junkmail subfolder, that has an x mark on it. It’s confusing that in the Mail preferences, it’s named ‘INBOX/spam’. I assume it’s Mail renaming the folder to Junkmail, as a result of it being set as the spamfolder, even though I’d expect a different name since I’m using the Dutch localized version. Since my Studio is always on or in sleeping mode, most spam ends up here.
I managed to set up my iPhone Mail so, that this is the ‘Move to junkmail’ folder as well.
Then there is a folder named ‘spam’ (without x) on the bottom of the list of sub mailboxes, that also has some spam mails in them.
Then there is another Junkmail (and x marked) folder listed onder On My Mac.
I know you didn’t make up this system, but I hope you can point me to an article or video that explains how we users can align all spamfolders to one point, or at least understand it, while considering multiple mailaccounts, mulitple devices including desktop / laptops, iPads and iPhones.
There’s some discussion of this in the manual. Each account has a special Junk mailbox (with the × icon), which is where SpamSieve will move the incoming messages. By default, it also moves messages there when you train them, although you can customize that.
The special mailbox will always be displayed as Junk (or the localized equivalent) even though the actual name on the IMAP server (e.g. your INBOX/spam) may be different (and may even be a subfolder). Generally, you want to choose as the special mailbox the same one that your mail server’s spam filter uses so that all the spam ends up in the same place. And you should choose the same one on each device.
You can optionally set up an All Junk mailbox in the Favorites section of Mail’s sidebar, which will combine (for display purposes) the contents of all the per-account Junk mailboxes.
It could be that you had SpamSieve set to move trained messages there. Or it could be that this is the mailbox that your server filter is using, in which case you should select it Mail’s Mailbox Behaviors settings for that account.
This could be the special mailbox for a POP account.
I never noticed the option to set the special mailbox before and I haven’t used POP in years, certainly not on my current two Macs or iPhone, but I guess Mail can still ‘see’ there once were POP accounts? On my MacBook, the preferences show a setting for ‘Junkmail: Op mijn Mac / Alle junkmail’ even though apparently there is not single spam mail ever send to it, since there is no such folder visible in Mail.
POP mailboxes are always stored locally, so I think they stick around if you delete the account. However, you can delete the On My MacJunk mailbox if you’re no longer using it.
You can choose to use the On My Mac one as the special mailbox for IMAP accounts, too, though I don’t recommend that.