Leave the Mac running all the time, so that SpamSieve is running all the time. That is how spam filtering is done for all mail clients and all devices sharing the mail account.
All spam filters take time to build up knowledge of what mail the recipient wants, and what mail the recipient doesn’t. This process is called “training”. There will be both false negatives (erroneously marked as not-spam) and false positives (erroneously marked as spam). Use Mark as Spam and Mark as Not Spam for misclassified mail messages, and the statistics-based filtering will adjust and become better.
Some mail providers do offer server-based spam filtering, some including server-based spam-filtering tools such as SpamAssassin. This approach avoids needing a local computer running full-time to filter for iPad and iPhone devices; for mail clients not running spam filtering. But SpamAssassin itself requires tuning and configuration.
A search for “remove SpamSieve” finds this: https://c-command.com/spamsieve/help/uninstalling-spamsieve
I’ve been using SpamSieve for many years (on a Mac that runs full time), and the occasional spam does still leak through. The spammers are highly motivated and only get paid when enough of their rubbish bypasses spam filters, of course. But SpamSieve does quite well, looking at all the rubbish in the spam folder.
For assistance with SpamSieve, contact SpamSieve Support.
