I've since done the following to try to change the envelope sender, and it seems to be showing up, so we'll see over time if there's any improvement.
define(`_REC_END_', `for $u; $|;
$.$b
(envelope-from: postmaster@$j)')

Latest issue was the new install, Thunderbird was not able to send, error:
Relaying denied: IP name lookup failed [192.168.0.34]
Simply needed to add to /etc/mail/access:
Connect:192.168.0 RELAY

I'm getting maillog errors:
sendmail: Losing ./qf#####: savemail panic
sendmail: SYSERR(root): savemail: cannot save rejected email anywhere
A super quick and easy way to test your postmaster alias:
date | sendmail -v -Am postmaster
This one turned out to be that I had define(`confLOCAL_MAILER', `cyrusv2') and MAILER(`cyrusv2') in my mc file. This was copied from another system that does work, I don't understand why it's working on that system...
I took that out, no more savemail panic, still not getting mail via imap though. Put it back, now suddenly everything is working. Don't know which other change I made that fixed it, but I'm gonna quit now while I'm ahead!

dnl Following limits size of file that will be sent to spamd, without this
dnl spamass will simply skip parsing of large emails and let them through.
define(`SMTP_MAILER_MAX', `15000000')
define(`confMAX_MESSAGE_SIZE', `15000000')


dnl Following changes from default: upped S and R from 4m to 6m, added T val to F switch

INPUT_MAIL_FILTER(`spamassassin', `S=local:/var/run/spamass-milter.sock, F=T, T=C:15m;S:6m;R:6m;E:10m')

define(`confMILTER_MACROS_CONNECT', `b, j, _, {daemon_name}, {if_name}, {if_addr}'})

define(`confMILTER_MACROS_HELO', confMILTER_MACROS_HELO``, {verify}'')
dnl 20140302 - getting msg Could not retrieve sendmail macro "b"!
dnl in logfile. see:
dnl http://lists.gnu.org/archive/html/spamass-milt-list/2006-05/msg00011.html
define(`confMILTER_MACROS_ENVRCPT', ``b, r, v, Z'')

define(`confINPUT_MAIL_FILTERS', `spamassassin')


dnl put in MAX_RCPTS_PER_MESSAGE after another customer send to hundreds
dnl of senders and it's not looping resending... 16-Aug-2010
define(`confMAX_RCPTS_PER_MESSAGE', `25')


vacation (autoreply)

• In user's dir you must create a .forward file, and a .vacation.msg file. Do web searches and man vacation for details on these.
• Be sure you don't have extra whitespace in .forward, e.g. "\foo, "|/usr/bin/vacation foo" does work, however, "\foo, " | /usr/bin/vacation foo" does not work -- you'll get a "550 5.1.1 User unknown".
• I was unable to tell if vacation -i did anything -- no ouput, no log event, no new directory, nothing.
• Be sure to do a man .forward too.
• For a virtual user (user in a virtual domain) I was unable to get it to work. My user does appear for local delivery in my base domain, but also has an alias in virtusertable. I did get it to work using the base domain, and as the virtual it did find the local user's .forward (I broke it, e.g. added the spaces described above and got the unknown user error), but was unable to get it to actually respond. I tried about ten million combinations of "-m", "-f", "-U", "-a", "-j", but no dice.
• Last thing I have is: "|/usr/bin/vacation -U -f /home/foo/.vacation.db -m /home/foo/.vacation.msg -j foo". It's not working, no autoreply, but now the email is no longer being delivered locally... Note that the standard example is \john, "| ...etc". After reading the manpage for .forward, I'm pretty sure I don't want the preceding "\john, ". I give up.

• Minor note to make AUTH work. in your /usr/ports/mail you'll find some sort of sendmail-sasl dir. Do the appropriate make incantations. Now, most important, you need to change the system default builtin sendmail over to the new one. FreeBSD has installed mailwrapper to do this for you. (Search for mailwrapper at http://www.freebsd.org/) Once this is done, you do not need to mess with /usr/local/etc/rc.d/sendmail.sh or with /etc/rc.conf.
• Use m4. Name your .mc file the hostname-of-this-server.mc
• Consider using the DNS name mail.domainname for mail handling. This allows you to move all mail handling to another server later by merely updating your MX records.
  Corollary: Do not configure a machine with the hostname mail.domainname as this would prevent you from bringing up a new machine, configuring it, and changing over at your leisure by merely changing MX records.
• Use the domain name with hostname for the machine name.:
  this is the confDOMAIN_NAME directive (specifically $j)
  this is very important when you set up your mail secondary machine.
  e.g. my primary machine (brix.vintners.net) has:
  

  define(`confDOMAIN_NAME', `brix.vintners.net')
  

  my secondary machine (vinifera.vintners.net) has:
  

  define(`confDOMAIN_NAME', `vinifera.vintners.net')
  

  Bear in mind that this means mail will only be answered when explicitly sent to this machine (host.domain). On the assumption that you want mail sent simply to the domain to get through, this is an additional domain name -- you will need to add the domain itself to the .cw file (see next step).
• If you wish for sendmail to answer to additional host names for this server:
  
  • Include them in the sendmail.cw (now local-host-names) file.
    e.g. I want my primary server to answer mail to mail.vintners.net so as to avoid ever having an explicit hostname in a mail path. Additionally, I don't want people to have to know that my mail server has a different name:
    sendmail.cw

    vintners.net
    mail.vintners.net
    

  • Be sure to tell sendmail to use the .cw file with:

    define(`confCW_FILE', `/etc/mail/local-host-names')
    FEATURE(`use_cw_file')
    

  • Add them to your MX records as lower cost routes:
    http://www.sendmail.org/~ca/email/english.html (see "mail loops back to myself")
• Bear in mind that sendmail does not deal with customer email readers -- you'll probably want to install popper and/or imap.

Primary machine

• If you are hosting multiple domains
  • You will need to include them in the .cw file.
  • Be sure .cw file is enabled. (See General above)

Secondary machine

• If you start by copying your primary machines .mc file, be sure to change the confDOMAIN_NAME ($j) value. If you don't, you'll get the totally non-helpful: "loops back to me" error. http://www.sendmail.org/faq/section4.html. Months down the line you'll have a terrible time trying to figure this one out.
• Cheap and sleazy
  • Put the main domain name in mailertable and have it point at the primary. This means that this machine will always pass all mail on to the primary for handling.
    If, for example, you were to install majordomo or mailman onto this server so as to offload list processing to an external machine, this configuration would not work as this machine would simply bounce the mail back to the primary instead of handling it.
  More delicate, but better (don't do this 29-Sep-2006)
  • Don't do the previous (do not put the main domain name into mailertable).
  • Do be sure to set up root with a .forward file to point to main machine. Use explicit primary name.
  • Do be sure to set up your main user (that you use to ssh into before su'ing) with a .forward to the primary. As with root, use explicit name to primary.
  • DO put this machine (host.domain) into sendmail.cw (local-host-names)
  • Make sure .cw file is enabled (see General above)
  • If you are hosting multiple domains, the secondary can be treated slightly differently. Simply add entries in the mailertable file to force all mail to said domain to be handled by the primary.
  The right way (29-Sep-2006)
  • Don't do the previous (do not put the main domain name into mailertable).
  • Do be sure to set up root with a .forward file to point to main machine. Use explicit primary name.
  • Do be sure to set up your main user (that you use to ssh into before su'ing) with a .forward to the primary. As with root, use explicit name to primary.
  • Use the confMAIL_HUB directive.
  • Be sure to include this machines name in the primarys local-host-names file.