Lots of customer emails in spam suddenly

I was wondering why the ticket number was so low in the last few weeks. Then I learned why.

Most of the tickets were marked spam.

I recovered most of them. But now I'm wondering:

1. what can I do to prevent this from happening in the future?
2. how many tickets have I lost? Do they delete themselves after some time? (if yes, are they recoverable?)

Thank you in advance for your help.