sendqmailq is intended for home computers that run qmail as their MTA but do not have permanent connection to the Internet. The outbound emails are stored locally in a queue, and when the IP connection is brought up, sendqmailq sends them to the ISP's SMTP server. Unlike serialmail, sendqmailq sends one single copy of an email to multiple recipients (instead of one copy each).
PHP Mobile Mail is aimed at mobile professionals who want to use their favorite MUAs (noteably pine or mutt) on their laptop, either in UNIX or Windows (under GNU Gygwin). This lets you work offline. It includes a mail_spooler program, which accepts the output of you MUA and spools (on, or offline). When connected, another process parses through the spool and sends your messages. It is also designed to "fake" using your REAL email address with free services like gmx.net, or mail.yahoo.com, when using their POP and SMTP servers, assuming you have a domain and account that you can control to this degree.
perl-esmtpd is an antispam, antivirus, and authenticating SMTP daemon. It integrates with qmail, sendmail, stunnel, etc. It's simple Perl, so you can hack it to suit your needs. It supports RBLs, mx on from, bounce evil phrases, virus scanning, etc. It offers: RBL-sytle black-hole DNS checking when not relaying, verification of valid MX records for the MAIL FROM (when not relayin, the ability to use Stunnel for smtps connections and still do RBL checks etc., simple authentication (i.e. PLAIN or LOGIN) for offsite users, and the ability to reject email with certain key phrases. Many other useful features are included.
Whoson (WHO iS ONline) is a proposed Internet protocol that allows Internet server programs know if a particular (dynamically allocated) IP address is currently allocated to a known (trusted) user and, optionally, the identity of the said user. The protocol could be used by an SMTP Message Transfer System in conjunction with anti-spam-relaying filters to implement a scheme similar to the one described here to allow roaming customers use their "home" SMTP server to submit email while connected from a "foreign" network.