When someone signs in to my custom domain admin console I schedule a follow up email in 7 days. It will either be a thank you or an offer of help, depending on if they set up a paid plan or not. I hate marketing emails, so I've tried to make this one as personal and 1 time sounding as possible, but with almost 20 signups a day but only maybe 2 conversions a week I felt it was necessary to have that touch point.
I'll likely add a 1 or 2 question survey to the email at some point, to try to understand why people do or don't follow thru with a custom domain plan, but for now I'll just see if the re-engagement makes any noticeable difference to the rate of subscriptions.