What commonly causes the problem of not receiving emails in infusionsoft?

What commonly causes the problem of not receiving emails in infusionsoft? Done checking the contact tags, done checking the email campaign and it was successfully sent and mark as unopen but when we open her Gmail we can’t find it. even in spam, promotions. What do you think is the problem?

That’s almost always an authentication problem. Without DKIM/SPF and DMARC setup on your domain server for sending emails with IS/Keap, you’ll either bounce or just be placed in the junk folder with most inbox providers. Otherwise, your email would be allowed in but just wouldn’t hit the inbox. So authentication is what manages that.