Re: fetchmail to an IMAP proxy with filtering?

On 2026-06-16, Jay Faulkner wrote:

> I've needed to improve my email situation for a while. Right now, I
> have my personal mail hosted on Office 365. I just IMAP it off into
> Thunderbird on my desktop, laptop, and phone. Two major problems with
> that setup right now: first, I keep getting rate-limited on IMAP from
> Office 365. Secondly, the filtering provided by the service is very
> limited and doesn't always work great.

This is not going to be a solution, but if you haven't yet, check
Thunderbird's concurrent connections setting for that account, and try
reducing it. I've, in the past, encountered situations where the
connection was refused because of too many connections (and IIRC even
the ones dropped probably because of a not-so-conformant IMAP
implementation (this being MICROS~1...) ended up being counted towards
that).

But take this with a grain of salt, because 1) this was with Exchange,
not really the centralized e-mail cloud offering in use with Office 365
nowadays; 2) the connection would be refused, not just rate-limited. But
I'm mentioning this in case it's worth a shot.

> So I was thinking, it's got to be possible for me to setup a kind of
> client-proxy, right?
>
> I'm thinking:
> - Fetch 100% of the messages/content/etc from upstream IMAP to my server
> - Use [local filtering mechanism of some kind] to filter them into
>   trash or specific folders. I'll note: spam filtering is handled pretty
>   well by Office365, so that's outta scope. This is more about putting
>   mailing lists in their proper folders reliably, and regaining some
>   power over things like github and gerrit e-mail notifications.
> - Host an IMAP server that I can then point at this filtered, downloaded
>   message content and feed it all to as many devices as I want without
>   fear of rate limiting.
>
> A couple of extra requirements I have:
> - Any sorting done locally (e.g. filtering mailing list posts to a
>   folder) needs to *also* happen on the upstream origin IMAP server. I
>   want it to remain the source of truth and for webmail to still be
>   useful.

It sounds to me you want something like offlineimap instead of
fetchmail-like, although I'm not entirely sure of how well does
offlineimap work with Exchange or the current Office365 offerings (I
recall having seen some issues once, but I'm not sure if that was with
Exchange, and whether that was caused by my side or the server's side;
might even have been something other than offlineimap...).

> - Ideally I can handle multiple accounts *with multiple logins*
>   transparently. Imagine right now I have a Thunderbird account pointed
>   to [email protected] at outlook.office365.com -- I want to just be
>   able to change that to personal_email_tld (or similar -- not a shared
>   login account) and have it transition over smoothly. If that's not
>   possible, I still want a solution where 1 login = 1 email account,
>   and not one where top-level folders map to specific accounts.

(This one I'm not sure I understand well, is this just about operating on
*all* accounts on the domain?)

> I'm asking on the list because... well, first of all I've never done
> this, but also, I just have no idea of the 2026 state of the art of
> these tools. I'm hoping someone can help. I've worked with email a lot
> generally, but all on the server/hosting side and all 15+ years ago so
> I know the protocols just maybe not the best tool choices.
>
> Any help you can provide is appreciated!

I'm sorry that I can't give more help, I'm just trying to mention things
that may be relevant, but I understand it will be just of little or even
no usefulness for your situation. I'm currently using a local IMAP
server but I'm not trying to mirror it, so I don't have recent
experience with that.

> Thanks,
> JayF
>
> P.S. I know there will be folks saying "change hosting". I'm not going
> to do that; as that would be disruptive to other family members on my
> domain. Either way, if I solve this myself the hosting becomes less of
> a concern generally.

As far as you're aware of stuff like smartscreen and the quarantine, and
the latter is properly monitoried to avoid messages being lost (I'm not
even sure if users get a notice if something is held in quarantine, or
if *all* refused messages go to quarantine, or if it's possible for
something to be silently dismissed).

-- 
Nuno Silva