Amavis will prepend to Subject (for local recipients only) if mail could not be decoded or checked entirely, e.g. due to password-protected archives.
To disable this on Centos/RHEL edit the file /etc/amavisd/amavisd.conf or on Debian/Ubuntu edit the file /etc/amavis/conf.d/99-warden and add the line (before the last line 1;):
$undecipherable_subject_tag = undef;
After making the changes restart Amavis:
// Centos/RHEL/CloudLinux/AlmaLinux systemctl restart amavisd // Debian/Ubuntu systemctl restart amavis