Email Thoughts 2 (reply to Tomasino and Sean)
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Thanks to both Tomasino [1] and Sean (email) for your super-helpful
comments and suggestions.

Tomasino, I like your idea of only allowing outbound connections to
whitelisted hosts, more-or-less for the same idea that I like the idea
of a gopherspace-specific mail system.  Of course, as you said, this
doesn't make much sense for my (currently) single-user server.  In my
case I suppose it makes more sense I think to maintain a whitelist of
servers from whom incoming connections are allowed.  The difficulty
would be in maintaining this list though...

Sean, sadly I can't reply to your email directly since, as I mentioned
in the last post, my bespoke mail server software doesn't yet support
outbound connections.  However, from what both you and Tomasino say it
sounds like I've been a bit too paranoid about thelambdalab.xyz
winding up on blacklists, particularly as I'm really only interested
in sending the occasional mail to other gophers.  (And to be honest, I
don't really care if Google blacklists my server.) So if I have some
time this weekend I'll implement a system for outbound connections
from lambdamail and just hope that the combination of SPF, DMARC and
properly-configured reverse DNS is enough.  (It seems I'll also have
to implement proper handling of SMTP 4xx responses: thanks for the
heads up on this, Sean.)

Thanks again for all the help!

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[1]: gopher://gopher.black/1/phlog/20190827-re-lambdalab-email-thoughts