Hey @marcin28! Are you using a verified sending domain now? If so, yes, it is normal for there to be an initial drop right after switching. But so long as you continue following healthy sending habits, it should be temporary!
The reason is because when you switch to using a verified sending domain (rather than one of ConvertKit's shared sending domains), you're essentially replacing your domain entirely. As a result, from email providers' perspectives (Gmail, Hotmail, etc.), you are a new sender. Therefore, your old domain reputation won't carry over once we set up the new one. This can cause a temporary drop in deliverability while email providers (re-)gauge your sending quality and recipients' engagement.
As a result, we would advise you to 'warm up' your new domain for about a week to build the reputation. You can start sending emails to your most engaged subscribers to begin with and gradually increase that over the week. (This is because as it’s technically a brand new domain you're sending from, if you're sending emails straight off the bat to a large amount of subscribers, that may have a negative impact.)
As long as you're sending high quality content, your deliverability should improve over the week! But a temporary deliverability drop is normal for the above reasons. We also have an article on verified sending domains here52 to help determine if using one is the best fit for your sending volume, etc. :)
Also, if you suspect that there might be another factor at play here, I'd recommend reaching out to our team at email@example.com (or via the in-app messenger in your account) with a description of the issue. One of our Product Specialists will be able to look at your account and help troubleshoot further!