Send email

I have a client who has two campaigns running, but in one campaign I get an error message when sending an email:

“Email send failed due to transport configuration issue.” If I delete the email address and add the same email address again to the other campaign, everything works perfectly. I tried to copy the campaign, but then I get a message saying I need to upgrade (I can't indicate or adjust this in the plans). So I deleted the campaign and recreated it, but the error message remains. So I have two questions: why does sending work in campaign 1 but not in campaign 2 (with the same email address), and why can't I just copy a campaign? I don't think I can adjust this in the plans.