Talvez porque não exista mais em 10.10 :
Package "dkim-filter"
WARNING: the "dkim-filter" package was deleted from this repository
Name: dkim-filter
Description:
DomainKeys Identified Mail (DKIM) Milter implementation
Implements a Sendmail Mail Filter (Milter) for the DomainKeys
Identified Mail (DKIM) standard. DKIM provides a way for senders to
confirm their identity when sending email by adding a cryptographic
signature to the headers of the message.
The dkim-milter implements both signing and verification.
Latest version: *DELETED*
Release: maverick (10.10)
Level: base
Repository: universe
Head package: dkim-milter
No entanto, eu encontrei este pesquisando um pouco. Basta baixar o arquivo .deb
e instalá-lo.