We'd like to look in to this further. You may need to check the spam settings via Post Office or you can also send us more details via private message of the email addresses that are being tagged as spam.
We also need to check if emails are being tagged randomly. We'll wait for your PM.
Similar problems. I recently changed from outlook 365 to Apple mail app as the Outlook 365 was always frying my laptop. I then set up Apple mail app. At first no problem then I couldn’t send or forward messages. I was given a message that the smpt server was offline etc. I looked up a fix that had me change the port numbers and untick a box in the settings. Some messages get sent others get labeled as spam