The default mail app on my Android HTC One requires ticking which folders to sync, other apps on other devices will probably have the same option.
Thanks gg30340 and licquorice.
I suspected that IMAP worked differently to POP, but it's reassuring to know that this is indeed the case.
I can see the Spam folders on my Android device which is running Marshmallow so henceforth I'll need to check them for messages as well as my several Inboxes. (Sadly Android assume a competent mail provider and don't appear to offer a Combined Spam Folder).
I have about half a dozen different email accounts on my phone, for each account I have just ticked which folders I wish to sync and then all mails are delivered to one screen with a different colour for each account.
Remember that anything you do on an IMAP account will be synchronised withe the BT servers. This means that if you delete an email from your phone it will be deleted from the BT servers and visa versa and will not be available to be downloaded by and POP3 account you may have on an other device.
Like wise if you send anything from your phone, it will show on the BT servers but not on POP3 devices.
Anything you send on POP3 will only show on that device and anything you delete from the POP3 account will only be deleted from that device.
I can't see on my phone, which is a Samsung Galaxy S7 running Android Marshmallow, how to control which e-mail folders sync with the server using IMAP, but my assumption is that they all do, including the Spam folder.
I will keep an eye on this and try to spot an e-mail in an account's Spam folder, and then move it to the corresponding Inbox. There doesn't howveer seem to be a means of display both the Inbox and the (supposed) Spam folder contents from all accounts in a combined view.
Thanks to gg30340 for the information on IMAP and for the link. what you've written seems very clear but I wll nevertheless study the article to make sure that I understand the posisble interactions when using multiple devices with a mixture of POP and IMAP.
That may be an incorrect assumption, on mine the default is to just sync inbox and sent folders. If you go to settings in your mail app, select the account there should be somewhere to select sync settings, possibly in advanced settings. On mine it is Sync, send and receive directly from selecting the account. If you prefer to use POP3, you can still set your phone up to use it, you are not forced to use IMAP if you don't want to.
One of my accounts I prefer to operate as POP3 with nothing left on the server and all mail held on my desktop PC where it can be backed up. I achieve this by having Outlook 2010 on the PC set to POP3 with the option to leave a copy of the message on the server unticked. On my phone, I leave the option ticked. I can then read the message on my phone but download it to my PC later when it will be deleted. I can delete it at any time on the phone without it being deleted anywhere else. I also automatically Bcc a copy of every sent mail from the phone to myself (another option on the phone app) so that there is a copy of the sent mail that can be downloaded to the PC. Hope that all makes sense.
When you open the email client on your phone, if you then tap on the "Inbox" with your email address at the top left this will open another "page" that will show you all the folders that are associate with that email account on the BT servers. You will see some other folders such as Junk Email, Bulk email etc that are set up by the email client. If you tap on the folders you will open them and be able to check the contents of them.
When you open/check the contents of the folders it will re-sync with the server.
Thanks to gg30340 and licquorice for the latest replies.
I have ascertained this morning that the Spam folder(s) on my Android phone are being sync'd with the server. I know this to be the case because I found an e-mail there, sent after I'd done my last POP download to the PC. I was able to move it to the corresponding Inbox on the phone, and to my delight it did not have a '[Bulk]' in the subject when I downloaded it to the PC (using POP).
Given how useless Yahoo's Spam filters are I'm happy that the Spam folder is sync'd, however this is just as well because I cannot see anywhere in the Android application settings how to disable this. Clearly I will need henceforth to check the Spam folder on the phone, which is a nuisance, but one must be thankful for small mercies. An obvious improvement in Android Marshmallow would be the introduction of a 'My e-mail provider is useless, display a combined Inbox AND Spam folder' option but I sense this won't be a development priority.
It's interesting that Android e-mail would also support POP but I think I'l stick with IMAP because it works well in conjunction with POP on the PC (POP being configured to not leave a copy of e-mails on the server), and also I'm reluctant to fiddle too much with the phone (it ain't broke, so it doesn't need fixing). I'm using e-mail across the fixed and mobile devices much as licquorice does: what was written made sense, and I see the implication is that, quite reasonably, syncing with IMAP is unidirectional (i.e. a snapshot of what is on the sever is conveyed to the phone via the IMAP protocol, so that the phone can only influence the contents of the Inbox of an account of which it has knowledge by CC'ing or BCC'ing the account on an outgoing message (which appears to be sent using SMTP)).