Log Message:
Version 4.87

 1. The ACL conditions regex and mime_regex now capture substrings
    into numeric variables $regex1 to 9, like the "match" expansion \ 
condition.

 2. New $callout_address variable records the address used for a spam=,
    malware= or verify= callout.

 3. Transports now take a "max_parallel" option, to limit concurrency.

 4. Expansion operators ${ipv6norm:<string>} and ${ipv6denorm:<string>}.
    The latter expands to a 8-element colon-sep set of hex digits including
    leading zeroes. A trailing ipv4-style dotted-decimal set is converted
    to hex.  Pure ipv4 addresses are converted to IPv4-mapped IPv6.
    The former operator strips leading zeroes and collapses the longest
    set of 0-groups to a double-colon.

 5. New "-bP config" support, to dump the effective configuration.

 6. New $dkim_key_length variable.

 7. New base64d and base64 expansion items (the existing str2b64 being a
    synonym of the latter).  Add support in base64 for certificates.

 8. New main configuration option "bounce_return_linesize_limit" to
    avoid oversize bodies in bounces. The dafault value matches RFC
    limits.

 9. New $initial_cwd expansion variable.