Updated sendmail packages that resolve an issue are now available for Red Hat Enterprise Linux 5.
Sendmail is a widely used Mail Transport Agent (MTA). MTAs deliver mail from one machine to another. Sendmail is not a client program, but rather a behind-the-scenes daemon that moves email over networks or the Internet to its final destination.
Sendmail previously used the MAXHOSTNAMELEN macro to allocate buffers containing host names and Fully Qualified Domain Names (FQDNs). The value of the MAXHOSTNAMELEN macro is set to 64 on Red Hat Enterprise Linux systems, which means that such allocated buffer can hold at most 63 characters, but according to the RFC1035, the FQDN maximal length is defined to 255 characters. This caused problems with a FQDN resolution in case that FQDN was longer then 63 characters. With this update, the code has been modified so that the size of buffers containing FQDN is now set to 256 bytes. The issues with the FQDN resolution now no longer occurs.
All users of sendmail are advised to upgrade to these updated packages, which resolve this issue.