opensmtpd-go-filter-prepend

OpenSMTPD filter which prepends a string on email's subject if not present.
Works with OpenSMTPD 6.6 and 6.7.

Usage

• build the filter for your target platform
  env GOOS=openbsd GOARCH=amd64 go build filter-prepend.go
• make OpenSMTPD use the filter

filter prepend proc-exec "filter-prepend-go --prefix='[*EXT*]' --extraprefix='[EXT]'"
listen on em0 tls pki "*" filter { senderscore, rspamd, prepend }

• default prefix is [*EXT*] if not specified on the CLI
• default extraprefix is [EXT] if not specified on the CLI
  Be warned that a too generic prefix could match legitimate wording and thus it won't be added to the Subject
• option --encode forces prefix encoding whether the subject is encoded or not.

Known limitations

1. if Subject is too long and spans between multiple datalines, only the begining will be analyzed for prefix token
2. RFC2047 Subject won't be decoded if 1/ happens inside an encoded string