F0 (zero)

F=0 (zero)

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Turn off MX lookups for delivery agent

V8.8 and above

During the delivery phase of a message, sendmail looks up the destination hostname with DNS and (possibly) redirects delivery to MX hosts , if present. One way (but not the best way) to suppress that MX lookup is to surround the destination hostname with square brackets:

 %  /usr/ucb/mail -v user@\[mail.us.edu\]  

Note that the square brackets are retained as part of the SMTP envelope:

 RCPT To:<user@[mail.us.edu]>   square brackets retained  

The F=0 delivery agent flag is another way to suppress MX lookups. To illustrate , consider using this delivery agent flag with the nullclient feature (FEATURE(nullclient)):

 FEATURE(`nullclient',`mail.us.edu') 

Here, all mail will be forwarded to mail.us.edu . To suppress MX lookups, we could surround the address with square brackets:

 FEATURE(`nullclient',`[mail.us.edu]') 

But this is unattractive and unnecessary. Instead, we use the F=0 delivery agent flag to achieve the same MX suppression effect:

 define(`SMTP_MAILER_FLAGS', `0')   prior to V8.10  define(`RELAY_MAILER_FLAGS', `0')   prior to V8.10  MODIFY_MAILER_FLAGS(`SMTP', `+0')   V8.10 and above  MODIFY_MAILER_FLAGS(`RELAY', `+0')   V8.10 and above  FEATURE(`nullclient',`mail.us.edu') 

Note that the F=0 delivery agent flag is suitable only for configurations such as nullclient . It can be extremely dangerous to use with any other delivery agents because it will cause necessary MX lookups to be skipped .