Comment 9 for bug 2056372

According to the exim4 changelog, the spf logic was modified in Aug 2023.

exim4 (4.96-17ubuntu1) mantic; urgency=medium

  * Merge with Debian unstable (LP: #2030098). Remaining changes:
     - Disable external SPF support to avoid Build-Depends on libspf2-dev
       (only available in universe). SPF can still be implemented via
       spf-tools-perl, as documented in exim4.conf.template. This reverts
       Vcs-Git commit 494f1fe, first released in 4.95~RC0-1.
       (LP #1952738)
       + d/control: drop Build-Depends on libspf2-dev.
       + d/EDITME.exim4-heavy.diff: disable support for libspf2.
       + d/d/c/a/30_exim4-config_check_rcpt: restore SPF logic based
         on spfquery.mail-spf-perl from spf-tools-perl, but without
         the previously supported helo detection.