Erm, qmail had lots of bugs[1], when compiled for 64-bit processors (lots of integer overflows), but djb pushed back and said 64-bit wasn't supported. If anything, qmail is known as the most annoying MTA to package, since no modifications to the source are permitted, and the application has to be built using a massive patch tree instead. The quirky management daemons required to run qmail were also obnoxious and at odds with everything else on the system.<p>Salient quote below:<p>>In May 2005, Georgi Guninski published "64 bit qmail fun", three vulnerabilities in qmail (CVE-2005-1513, CVE-2005-1514, CVE-2005-1515):<p>[snip]<p>>Surprisingly, we re-discovered these vulnerabilities during a recent qmail audit; they have never been fixed because, as stated by qmail's author Daniel J. Bernstein (in <a href="https://cr.yp.to/qmail/guarantee.html" rel="nofollow noreferrer">https://cr.yp.to/qmail/guarantee.html</a>):<p>>>"This claim is denied. Nobody gives gigabytes of memory to each qmail-smtpd process, so there is no problem with qmail's assumption that allocated array lengths fit comfortably into 32 bits."<p>1. <a href="https://www.qualys.com/2020/05/19/cve-2005-1513/remote-code-execution-qmail.txt" rel="nofollow noreferrer">https://www.qualys.com/2020/05/19/cve-2005-1513/remote-code-...</a><p>edit: added quote from referenced url