Filters that move from IMAP accounts to local folders can be troublesome. This partially about how IMAP works, and partially an area of Thunderbird has has some issues. In the first instance try making sure all filter execute on IMAP accounts is after classification. This I think delays the execution for just a bit and allows the imap sync of the mail to happen. Second as you have tried, copy not move. Then add a delete for the email to clean up. This more closely resembles the IMAP specification as when you move to "local folders" you are removing the mail from the IMAP account (deleting) and adding it to the local folders account. In there is a round trip to the imap server to validate the mail as complete, then another to delete the mail. SO making it explicit and not having software try and translate your intention is a better approach. Additionally make sure you do not have an "security" software scanning mail. Most of the odd failures to delete, or copy to another IMAP account come from silent failures in the synchronization process because it is simply too slow completing. Having the overhead of scanning to further slow what is a relatively slow process only makes the problem worse. Just be aware, Thunderbird receives text, and stores text. Scanning is really of little import until you open the mail or attachment when attachments stop being text. Keep your filters trim. Each new mail is run through the filters in your list sequentially. So if the same email meets multiple criteria, thing can get slowed down. Just as filters that have long lists of if the address contain or is "whatever" really slow execution as each condition must be checked one after the other. Copy IMAP mail and delete it in the same filter. Once a mail is deleted it leaves the list of new mail to be run through the remaining filters.