Once the first host or domain specification has been extracted from the address, the MTA consults the rewrite rules to find out what to do with it. The host/domain specification is compared with the pattern part of each rule (that is, the left side of each rule). The comparison is case insensitive. Case insensitivity is mandated by RFC 822. The MTA is insensitive to case but preserves it whenever possible.
If the host or domain specification does not match any pattern, in which case it is said to “not match any rule,” the first part of the host or domain specification—the part before the first period, usually the host name—is removed and replaced with an asterisk (*) and another attempt is made to locate the resulting host or domain specification, but only in the configuration file rewrite rules (the domain database is not consulted).
If this fails, the first part is removed and the process is repeated. If this also fails the next part is removed (usually a subdomain) and the rewriter tries again, first with asterisks and then without. All probes that contain asterisks are done only in the configuration file rewrite rules table; the domain database is not checked. This process proceeds until either a match is found or the entire host or domain specification is exhausted. The effect of this procedure is to try to match the most specific domain first, working outward to less specific and more general domains.
The host/domain specification is used as the initial value for the comparison strings spec_1 and spec_2. (For example, spec_1 = spec_2 = a.b.c).
The comparison string spec_1 is compared with the pattern part of each rewrite rule in the configuration file and then the domain database until a match is found. The matching procedure is exited if a match is found.
If no match is found, then the left-most, nonasterisk part of spec_2 is converted to an asterisk. For example, if spec_2 is a.b.c then it is changed to *.b.c; if spec_2 is *.b.c, then it is changed to *.*.c. The matching procedure is exited if a match is found.
If no match is found then the first part, including any leading period, of the comparison string spec_1 is removed. Where spec_1 has only one part (for example, .c or c), the string is replaced with a single period, “.”. If the resulting string spec_1 is of nonzero length, then you return to step 1. If the resulting string has zero length (for example, was previously “.”), then the lookup process has failed and you exit the matching procedure.
For example, suppose the address email@example.com is to be rewritten. This causes the MTA to look for the following patterns in the given order:
sc.cs.siroe.edu *.cs.siroe.edu .cs.siroe.edu *.*.siroe.edu .siroe.edu *.*.*.edu .edu *.*.*.* .