Are these different accounts completely separate Fastmail accounts where the spam is send directly to the main address or an alias at that account? Or are you discussing several aliases at one account, or an account at a non-Fastmail provider, or emails forwarded (or fetched) from a non-Fastmail account?
Have you gone to the trouble of examining the sending IP address and other details on the similar spam received at the two accounts? Spammers often switch the sending systems to get around spam filtering, and it's quite possible that the spam messages you receive which superficially appear to be similar were sent from different servers and have quite different spam signatures. Spam is quite varied, and unless you are getting slammed with many messages each day which are very similar, it's just not a good use of your time to mess around with custom filtering.
I think that only reasonable solutions are:
- Using the normal spam filtering systems separately on each account. See:
https://www.fastmail.com/help/receive/stopspam.html
If you have learned at least 200 messages as spam and 200 as non-spam, a personal spam filter will be activated automatically for that account. Because I have reported a large number of good message as non-spam, I get very few false positives and have set my spam threshold down to 4.0 for moving to the Spam folder. This setting works well for me, but might not work for others.
- If you get a large quantity of spam with similar characteristics (such as message content in the subject or body, sending IP address, sending country, etc.), you can create a rule to force such messages to be directed to your spam folder.
- The easiest way to start on this is to read such a spam message using the Fastmail web or mobile phone client, then do the following:
- Click More ... Add rule from message ...
- Edit the conditions at the top. Change, add, or remove conditions if they won't trigger on the spam messages.
- Check "Sent to spam".
- You can give this rule a name so you can find it later.
- Click Save.
- You can go back and edit this spam rule to add or modify the conditions.
- In difficult cases, it's possible to add custom sieve code, but with the current system I think this is a waste of your time. The spam system works well if you let it work, and learn >200 spam and non-spam messages (and any commonly received messages) so that your account can learn your situation.
- Note that received messages are not automatically learned as spam, even if they are auto-filed into your Spam folder by the default system spam filter. You need to force them to be learned as spam after making that decision yourself. I have a folder which is normally empty I created called "SpamReporting" and if I start getting a lot of spam I move those messages into that special folder, then select all messages in that folder and mark them as spam. They will then be automatically moved back to the Spam folder and learned as spam.
- Although you could look at the spam messages received at one account and create spam filtering rules at the second account based on the characteristics, this is usually just not worth the effort. Let the existing spam filtering system do it's job.
- I would not forward spam messages from one account to another. You would be risking being classified as a spam sender, and the receiving system may blacklist that Fastmail outgoing SMTP IP or your account. It's always best to mark as spam and then delete incoming messages on the email account which directly received the spam, and never forward a spam message.
Bill