It's been a while now and since then I have sent plenty of e-mails. Sure enough, the open rates have significantly decreased. I made a little test with a new list as well and the results were not good at all.
I have read another thread where it was claimed the on average, open rates went down by 7%. The percentage is much higher for me... on average, the open rate is 20% less than it was before. That's a serious decrease! I made tests with a new responsive list as well and the results were not good.
The increase in solo ad sellers and list builders in general probably contributed to this decrease as well but I'm pretty certain the Gmail update is the main cause. After all, a lot of my subscribers use Gmail accounts.
E-mail marketing is definitely still profitable but this is a serious blow.
What do you think about all of this? Did your open rates decrease as well?