See Table 20-1 on page 717 for a guide to
all delivery agents. And note that when $u appears in an A= equate, it should always be last
because there might be multiple recipients.
In general, the definitions in the cf/ostype subdirectory are pretuned in a way that is best for
most sites. If you want to make changes, remember that each definition that you put in
your mc file replaces the definition in cf/ostype. Therefore, it??™s best to copy an existing definition
and modify it for your own use. Just be sure you don??™t omit something important.
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20.5 Delivery Agent Equates | 739
20.5.2.2 The use of $h in A=TCP
For network delivery via the P=[IPC] delivery agent, the A= delivery agent equate is usually
declared like this:
A=TCP $h
The value in $h is the value returned by the parse rule set 0??™s $@ operator and is usually the
name of the host to which sendmail should connect. During delivery the sendmail program
expands this hostname into a possible list of MX records.* It attempts delivery to eachMX
record. If all delivery attempts fail and if the V8 FallbackMXhost option (?§24.9.48 on page
1030) is set, delivery is attempted to that fallback host. In all cases, if there are no MX
records, delivery is attempted to the A or AAAA record instead.
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