Email bombs: How can I defend a web email account against one?

For what reason?

To save email addresses from an email bomb targeted at Office365 and Outlook.com accounts using custom domain addresses (we use/pay for both biz and personal accounts). Targeted accounts were signed up for thousands of newsletters, catalogs, etc. Inboxes are flooded daily. Without a practical solution, years held long term email addresses would have to be given up.

Probably not solutions

  • Manually blocking addresses one at a time. It's slow and tedious. So far I can only find a small manually maintained blacklist, which is not practical due to time required or item count limitations (cannot see a way to import a large 5000 item list).

  • I already have a scrubbed/clean list to be blocked (unique addresses/domains only) as a text file. The size of addresses to block seems to taper off eventually, which is why I think this problem is solvable.

  • Features using Bayesian ratings and/or machine learning. The attacker simply signed up accounts to receive tons of email. A lot from normal businesses that shouldn't be flagged.

Possible solutions

  • Any combination of features that could create an effective 5,000+ item blacklist of addresses/domains. Doesn't have to be only one specific capability.
  • I've researched and found Sieve Scripting, which is a powerful Internet email RFC standard. However, the feature is not well documented and often not even listed as an available feature.
  • Add-ins/Plugins. I've searched for plugins to leverage as well but so far strangely haven't found nearly any. (KuTools doesn't work well for this).

More information

  • The reason I only need to figure out how to block 5000, is it seems the attack is large, but finite.
  • 1
    Hi whitneyland, recommendations are off-topic on the site. Software Recs is a possibility, but please check with their meta as to whether service recommendations are on-topic (I seem to recall that they are not, but that might have changed). That being said, it's hard to tell whether you want a new email service for the affected members to be able to switch to rather than how to block this activity on their existing ones. If they want to remain with their existing providers, checking with those services to see if there's some way to block this on the SMTP server might be an option. – jonsca Mar 17 '19 at 0:15
  • @jonsca Can it be taken off hold now? I have rewritten the question to be purely about how to accomplish a task with a web app. It should have zero trace of any request for a recommendation of a service provider or app. Please advise if it is now satisfactory, or if I missed anything. If I have complied fully in letter and in spirit it would also be nice to have your +1 vote reflecting this new compliance with the site's mission. – whitneyland Mar 17 '19 at 1:26
  • The question should not be reopened as it's unclear as it doesn't specify which specific web app is being used. Relate Q/A on meta How do I ask a question that may require recommending a web app? – Rubén Mar 17 '19 at 4:37
  • 1
    @whitneyland Thanks for improving your question to meet this site guidelines. I voted to reopened it. I didn't use Office365/Outlook admin tools for something like you are trying to do yet. Regarding Sieven Scripts the Stack Exchange search returns 0 results today (stackexchange.com/search?q=sieven+scripts) – Rubén Mar 17 '19 at 17:07
  • 1
    @whitneyland All set. – jonsca Mar 17 '19 at 22:34

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