Although a class macro name can be any ASCII character [6] (any character in the range 0x0 to 0x7f), avoid using any of the nonletter characters . At the very least they create confusing reading, and at worst they can cause sendmail to completely misinterpret your intentions.
[6] Other than the { character.
Although values can traditionally be made to contain whitespace by quoting them, class macros will misinterpret those quotes. For example, "vax ds1" wrongly parses into two class entries: "vax and ds1" , with the quotes a part of each.
Duplicate values are silently ignored. Therefore, typos in a list of values can cause an accidentally duplicated entry to be silently excluded.
Avoid creating a new class macro name without first checking to see whether it has already been used. That is, don't create a list of UUCP hosts within class $=U without first checking both for preexisting CU and FU definitions and for rule-set uses of $=U and $~U . It is perfectly legal for the $=U and $~U expressions to exist in rule sets without a corresponding CU or FU definition. However, such empty references will still cause sendmail to search the string pool.
Under V8 sendmail you can watch your class macro definitions being formed by using the -d37.8 debugging switch (-d37.8). Under other versions of sendmail you can only approximate this information by using the -d36.9 debugging switch.
The file form's scanf (3) pattern can produce unexpected results. Remember that the pattern is applied to a line, not to a stream.
No error checking is performed during reads for the F form of the class configuration command. An I/O error reading from a file silently causes the rest of that file's contents to be ignored. An unreported error from a program (one that silently returns 0 on both success and failure) is also silently ignored by sendmail .
