From: Wietse Venema on
Stan Hoeppner:
> Victor Duchovni put forth on 9/26/2009 1:36 PM:
> > On Sat, Sep 26, 2009 at 05:36:30PM +0200, Sven Strickroth wrote:
> >
> >> Am 26.09.2009 16:18 schrieb Wietse Venema:
> >>> OK, so you send out IP packets with your static IP source address
> >>> out over the dynamic interface.
> >> no, and there's the problem. if the static-ip connection is down,
> >> postfix reports "warning: smtp_connect_addr: bind x.x.x.x: Cannot assign
> >> requested address" and then it binds to the dynamic ip and uses that.
> >>
> >> Why not add an option so that if the binding to the smtp_bind_address
> >> fails that postfix refuses to send mail.
> >
> > Add filters to block outbound port 25 on the dynamic interface. Pretend
> > your ISP has already done that for you (as many do).
> Or...
> Might it be possible, via transport_maps or something, to relay via SASL
> auth to your dynamic IP DSL provider's submission/outbound SMTP relay
> server, either on TCP 25 or 587 (whichever they support), whenever your
> static IP interface is down? This will definitely help to mitigate some
> delivery issues you mentioned WRT receivers' anti spam measures. I ran
> this way for almost 4 years on an SBC/AT&T PPPoE dynamic IP DSL
> connection with great success, although it was a single line setup.

Postfix supports smtp_fallback_relay (set it to the dynamic ISP);
and Postfix SASL passwords are always tied to specific SMTP servers.