Some mitigations mentioned in https://lists.exim.org/lurker/message/20190906.185037.1ff8bb...:
> Add - as part of the mail ACL (the ACL referenced by the main config option "acl_smtp_mail"):
deny condition = ${if eq{\\}{${substr{-1}{1}{$tls_in_sni}}}}
deny condition = ${if eq{\\}{${substr{-1}{1}{$tls_in_peerdn}}}}