The receive_msg function in receive.c in the SMTP daemon in Exim 4.88 and 4.89 allows remote attackers to cause a denial of service (infinite loop and stack exhaustion) via vectors involving BDAT commands and an improper check for a '.' character signifying the end of the content, related to the bdat_getc function.
https://github.com/krlabs/eximsmtp-vulnerabilities
https://lists.exim.org/lurker/message/20171125.034842.d1d75cac.en.html
https://euvd.enisa.europa.eu/vulnerability/EUVD-2017-8113
https://bugs.exim.org/show_bug.cgi?id=2201
http://www.openwall.com/lists/oss-security/2021/05/04/7
http://openwall.com/lists/oss-security/2017/11/25/3