In-Active domains are not charged and refer to domains that have no MX records and no active subdomains. Instead, a defensive SPF record must be published where the DNS syntax of the defensive record is:
Type: TXT
Host: @
Value: v=spf1 -all
Your in-active domains MUST NOT use any of the following features: Hosted MTA-STS, Hosted TLS-RPT, Hosted BIMI, or PowerSPF to be considered as in-active domains.