Hi,

By default message.el uses mail-abbrev to expand mail aliases in
headers in a draft.  If an aliasee is longer than fill-column,
it will be expanded like the following:

To: foo1@bar.baz, foo2@bar.baz, foo3@bar.baz, foo4@bar.baz,
    foo5@bar.baz,
    foo6@bar.baz,
    foo7@bar.baz,
    foo8@bar.baz

To reproduce this, try the "foo" alias specified as follows:

(define-mail-abbrev "foo"
  "foo1@bar.baz, foo2@bar.baz, foo3@bar.baz, foo4@bar.baz,\
 foo5@bar.baz, foo6@bar.baz, foo7@bar.baz, foo8@bar.baz")

The point is that the latter half addresses in an aliasee are
put line by line.  Moreover, I think a SPC is enough for LWSP
preceding to the latter half (message.el uses TAB in some fill
function, though), rather than the one indent-relative generates.

A patch I tried is attached below.