Returned email/wrong or old addresses

I’ve come to v3.4.9 from a very old version and everything seems to be working okay - I did a new campaign and it appears to be in process.

The only difference is this. Under the old system I got a bounce back email/error for every address in the DB that was wrong, out of date etc. This time it’s not happening.

There are always a large number of them as the DB has 6,000+ names in in. The ‘from’ email address I used when I set up the campaign is correct. Have I not selected something I should have or done something wrong?