> "sender=list" appears to have done the trick (messages are no longer
> flagged as suspicious by hotmail).

Right -- issues with "On Behalf Of" aside, Sender=List is always your best
bet for delivery to Microsoft sites.  The reason is that Microsoft's
SenderID Framework (SIDF) "improves" (*cough*) upon SPF by taking into
account the Sender: and From: headers of a message prior to evaluating the
message return path (MAIL FROM).

For more information about LISTSERV, SPF, SIDF, and other fun stuff
(DomainKeys), see the following paper:

http://www.lsoft.com/news/techtipLSV-issue3-2006-us.asp

(Footnote 2 talks about the Sender: header and its relationship to SIDF.)

For my money, you're usually better off using Sender=List and dealing with
complaints about "On Behalf Of" from Outlook users than using Sender=None
and dealing with complaints about mail not being delivered to MSN users.
Pick your Microsoft-induced poison :)

And for the record, Sender=List is the documented default for LISTSERV.

--
Liam Kelly
Senior Consulting Analyst
L-Soft international
[log in to unmask]

L-Soft is proud to offer on-site or remote
Training and Consulting Services.  For details, visit
<http://www.lsoft.com/products/tcservices.asp>