What files are you referring to - backup files? Or profile data? (You shouldn't have your profile data on a cloud device)

It's the main files that store all the emails. My profile data is on my personal PC, but I back up my PC to a cloud backup service. Because the email store files are so big (Gigabytes!) it takes a while to upload at 1MB/sec. If the email store files were broken into smaller segments (100MB is a nice size) and new ones added as each reached it's maximum size, then my backups would only need to upload the newest file.

Any movement on this feature request? (it's ben a long time, but I couldn't remember which account I used to post the initial question)

You could make the jump to maildir. https://support.mozilla.org/en-US/kb/maildir-thunderbird It has some bugs I understand. https://bugzilla.mozilla.org/buglist.cgi?classification=Client%20Software&classification=Components&f1=short_desc&f2=OP&f3=short_desc&o1=nowordssubstr&o3=anywordssubstr&product=MailNews%20Core&product=Thunderbird&resolution=---&short_desc=maildir&short_desc_type=allwordssubstr&list_id=17151813

However it does reduce the file sizes of everything to the size of a single email. Other that the global index, and it should not be backed up anyway as Thunderbird can rebuild it given the mail to search through.

https://support.mozilla.org/en-US/kb/rebuilding-global-database

Thanks for the advice Matt. Maybe I'll use maildir when it's ready.

Anyway, my feature request isn't just for my benefit. Think of all the individuals and companies uploading gigabytes of data to the cloud or off-site backups. All those bits are creating extra traffic on the internet. There' enough clutter on the internet as it is. I can't remember the actual figures, but I recall reading that spam and junk email accounted for a significant proportion of internet traffic.