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письма с форума стали приходить в папку СПАМ. Как от этого избавиться.»

Попробуйте сначала стандартные рекомендации GS по настройке фильтров:

Create some filters for a few of the senders who have been going to spam. Use the "never send to spam" option on those filters. ALSO, have the filters add a new label you create. Then, the next time those sender's messages end up in Spam check to see if the new label is applied to them or not.

раньше всё было нормально

Most of the major email providers, including Yahoo!, AOL, Hotmail, and Gmail, use engagement-based filtering to help detect spam. This means that the more your recipients interact with your emails by opening, clicking, and reading, the more highly engaged your audience is, and the more likely you'll end up in their inboxes. If a lot of users are marking a message as spam, it's more likely to end up in other people's spam folders. If you've moved emails out of the spam folder, that's a positive indicator and can help ensure future emails that are similar are delivered to the inbox instead.