sendmail, 4th Edition

Look Up Addresses for Delivery

When sendmail prepares to connect to a remote host for transfer of mail, it first performs a series of checks that vary from version to version. All versions accept an IP address surrounded with square brackets as a literal address and use it as is.

Beginning with V8.1, sendmail first checks to see whether the host part of the address is surrounded with square brackets. If so, it skips looking up MX records. (We’ll elaborate on MX records soon.)

Beginning with V8.8, sendmail first checks to see whether the F=0 flag (F=0 (zero) on page 761) is set for the selected delivery agent. If it is set, sendmail skips looking up MX records.

If sendmail is allowed to look up MX records, it calls the res_search(3) BIND library routine to find all the MX records for the host. If it finds any MX records, it sorts them in order of cost, and lists them, placing the least expensive first. If V8 sendmail finds two costs that are the same, it randomizes the selection between the two when sorting.^[152]

After all MX records are found and listed, or if no MX records are found, sendmail adds the host specified by the FallbackMXhost option (FallbackMXhost on page 1030) to the end of the list. For V8.11 and earlier, the hostname, if there was one, was added to the end of the list as is. Beginning with V8.12, if a hostname is listed, MX records are looked up for it as well, and those MX records are added (in the proper sorted order) to the end of the list. By surrounding the hostname ...