The short answer is you do not. Thunderbird does not have that Eudora feature.

You could install FiltaQuilla, then create a filter to automatically save or detach attachments to an external folder.

Many thanks to both Matt and sfhowes.

Matt confirmed what some months of use had led me to believe but sfhowes gave me hope that there was an Add-0n that solved the problem. I have tried FiltaQuilla but so far have been unsuccessful in getting it to Autosave attachments the way Eudora did.

Even if I could get it to work I am not sure that it would not simply create another problem. In Eudora all attachments were saved to disk but remained attached to the email they arrived with. This kept the email folders to small sizes as the bulk was in the attachments. If you deleted an email its associated attachment would also be deleted. Great system.

I find however that in Thunderbird once you have saved an attachment whether manually or otherwise it will not be deleted when you delete the associated email. It is not completely detached from the email because you can still open it from the email as long as you have not set saving it as a default operation for that file type, but it is not sufficiently attached to be deleted when you delete the email. It is therefore best not to save the attachment until you know you want to keep it.

I greatly lament the loss of functionality that came with Eudora but sadly it really didn't work fully with modern operating systems and IMAP so I have had to change. I think I will just have to carry on saving attachments manually.

Deleted duplicate reply here

Why would you save the attachments if you don't want to keep them after you delete the original email message?

Fabian said

I greatly lament the loss of functionality that came with Eudora but sadly it really didn't work fully with modern operating systems and IMAP so I have had to change. I think I will just have to carry on saving attachments manually.

There is a security issue here. It is not large, but present none the less. Attachments included with an email are in encoded MIME. This means that regardless of what malware or other nasties that might b included in that email, they are completely safe as MIME is a non executable data format. By leaving attachments in MIME, Thunderbird side steps the underlying issue of the malware getting into the loose as it converts the MIME encoded text back to it's binary form upon receipt. There is also no Binary file anywhere for someone to accidentally open when they are searching. How often have you opened a file just to see what it contained? I do it often when something is misplaced. Those binary attachments just do not get the same care and wariness when they are already on your hard disk. At a purely human level I mean. We tend to trust what we already have.

With modern hard disks having an entry level in the TeraByte range, A few kilobytes for a mail attachment looses a lot of it's importance. With Thunderbird finally looking like it has shaken the 4Gb limit per folder the efficiency of making mail folders small has almost no value. The disk will be big enough and the software does not care, it uses an index to find mails in the folder anyway.

With the caveat that "there be bugs in here", the new Maildir in Thunderbird storage format makes the separation of large attachments even less useful as backups will back up a single file per email rather than a monolithic mail database, so incremental backups will see a huge performance enhancement.. A 10Kb email will no longer trigger a multi Gigabyte backup.

That's a really helpful explanation Matt, Thank you.

Backups and Synchronisation were two of my concerns about large mail files and this encourages me that my concerns may be misplaced. My experience of very large files is also that they used to corrupt causing severe loss of data but what you say about size capacity makes me feel better about that. my example is Outlook which I do not use because it keeps everything in one humungous file. I have heard too many stories of people losing their history when that corrupts.

Matt said

With the caveat that "there be bugs in here", the new Maildir in Thunderbird storage format makes the separation of large attachments even less useful as backups will back up a single file per email rather than a monolithic mail database, so incremental backups will see a huge performance enhancement.. A 10Kb email will no longer trigger a multi Gigabyte backup.

I see that Maildir is experimental in Thunderbird 38 and will not impact on existing folders. I will try it out having just updated to 38 but as it only applies to new email accounts I am not sure that it will impact much on me unless that means new folders

No it means new accounts. although you can change the settings and then reset up an existing account. I have not had time to play with it myself, but I am assuming that existing MBOX files can be exported to the appropriate folder an EML using the import export tool's add-on.https://addons.mozilla.org/en-US/thunderbird/addon/importexporttools/

Thanks again Matt,

Having re-read the info about MailDir I am not sure I have the courage to switch to it yet. I have some other changes to make just now and the first is to switch from POP to IMAP so I think I will bed that in first.

When I do eventually make the switch the information you have provided will be very helpful.

My wife's email is on IMAP and the file structure there looks as though it may already have 1 email per file though until I looked at her version Number she was only on Version 31.7. Looking at About seems to trigger an update. She has just updated to Version 38 so I may try MailDir on her account first. Do you know if MailDir will cause a change in structure to IMAP folders if I switch to it before enabling IMAP on my account.

I am just backing up her IMAP folders. My backup was not doing them before. She has a folder called GMail.sbd\AllMail.mozmsgs which looks in Windows Explorer as though it is lots of different files but backs up as though it is one file and takes forever so I take that as an indication that if I am going to switch to MAilDir I should do so before switching to IMAP.

I the option to allow windows eeach to search mail is selected then an additions WDSEML file is created for that purpose. Identical to an email but missing attachments. I disabled the option and deleted the files and recovered quite a few Gb of space that was wasted because I simply try and avoid windows search for anything but files. I figure Thunderbird does a plenty good enough job of searching mail for me.

Yet again you have the answer.

I had been investigating these files further because they are proving a nightmare to backup and I realised that there are no new WDSEML files in mozmsgs folders since I imported from Eudora to Thunderbird. The option for Windows search must have been enabled then but is not enabled now and I have no such files or folders in my email system. If I can get them backed up I will delete them as they sure do take up a lot of space.

Many thanks for your continued help.