The problem is your SMTP server is not accepting the request, it most probably has to do with rate limiting by the server. Some of the less forward thinking providers, particularly in the USA use rate limiting to force the use of commercial account instead of domestic for small business.

In the case of charter, spectrum have some obvious limits. Such as the following;

• You can send messages up to 30 MB in size, including attachments.
• Attachments can't exceed 20 MB.
• You're allowed to send emails to up to 1,000 recipients every 24 hours.
• Your emails can have up to 99 recipients per message (including the To:, Cc: and Bcc: fields).
• Email accounts less than a week old have a sending limit of 100 emails per day.

https://www.spectrum.net/support/internet/charter-postmaster-services

Given IMAP and POP are incoming protocols and SMTP is the outgoing protocol for both, perhaps your IMAP solution would work. But I don't see how to limit the number of sends, except by not using filters.

You might want to look at automatic forwarding on the server if you want to forward every mail to some other address.

Thanks Matt for the useful information about Spectrum e-mail! I checked but Spectrum doesn't appear to offer automatic forwarding (actually I need automatic filtering). I would also note that I have had filters in place for years but the problem has only cropped up recently. It's entirely possible that Spectrum has changed their protocols to prevent simultaneous SMTP connections. This is more likely as you point out. I may have to resort to manually forwarding messages, one at a time. Thanks again!

Perhaps filter to a folder (copy) then bulk forward them once a day and delete the copies once forwarded.

Thanks Matt! I may have an alternative solution. I can create another e-mail account (call it "distribute") and have some e-mails from the outside world go there. Spectrum allows one to forward e-mails (to 1-5 addresses). I will need to reconfigure the sending sites to send to "distribute" which can then be forwarded to my real e-mail address and others. No need for filtering at all. If this works, I will follow up with another reply.

The above alternative solution worked fine...