Has two uses - both checks...

• does a variable have a value, i.e. not null or blank
• does a specific header exist in the message

Checking a variable for a value...

${if def:sender_host_address {remote} {local}}

Returns remote if the $sender_host_address is not empty.

Note the absense of the $ character.

Checking for a header...

${if def:header_reply-to:{$h_reply-to:}{$h_from}}

Returns the value of the reply-to header if it exists otherwise it returns the value of the from header.

Compairs two strings for equality after expanding them...

eq{String-1}{String-2}

Returns true it the two strings match and false if they don't.

${if \
   eq{$local_part}{john} \
   {true} \
   {false} \
}

If the local_part matches john return true.

NOTE: the match is case sensitive.