I upgraded to version 10 the other day and right away I have encountered lots of issues. first no incoming mail goes into any inbox. only shows up in the unread folder. No where else.
Sounds like when you upgraded to V10 you chose “the default install” which then categorises all your incoming and existing mail using an algorithm to sort in Inbox mail folders such as eg: Primary, Promotions, Social, Other, Updates etc.
So look on the right of the Primary Inbox folder and see if you have other different named Inbox Tabs. If you do, then click the link below to see how to move messages back to the Primary tab or any other tab you want for those sender messages in future.
You can also disable any Category tabs you don’t want or disable the Categories completely.
Another issue is I have tried to reply to a mail from a company answering an issue from them. it refuses to send with the error saying that it could no send because the message had spam in the message. looking at the error did not indicate where the offence was.
If you are replying to a senders email and you get an error sending due to spam in the message “that would be impossible for us to know why”. Could be anything in the senders mail message.
Normally if I get an email like that I personally just delete them and advise the sender if I know them.
In alot of cases senders don’t even know that there is spam or viruses etc in emails they send. Could be your Antivirus / Malware resident program is scanning the outgoing mail and found some spam in it, or if the message bounced back from your Sending Mail server, they might have also scanned it for suspicious activity and blocked it going out.