I'm sure I've done this (and sometimes feel I've forgotten more than I remember).
I want to automate the sending of certain reports in a campaign. One at 1 day after and email send, and the other 5 days after the email send.
The subscription model isn't this granular, but I feel sure it can be done.
I guess I mean MailBoost more than FlowBoost alone. But you could use AWS SES or equivalent directly if you wanted.
The idea is to