What is the retention policy on your imap server for sent mail?

According to my web host, "We don't enforce any specific policy on how long emails are kept - they are kept indefinitely. Typically when emails get deleted, we find that it's either a mail client or mobile device doing it."

Check Thunderbird imap account settings:

• Right click on imap mail account name in Folder Pane and select 'Settings'
• Select 'Synchronisation & Storage'

Under Disc Space... If you have a setting here that says to delete messages, then this will also be effecting any folder. Therefore you would need to specifiy the 'Retention Policy' on certain folders if they are not to use these settings.

Otherwise:

• Select 'Don't delete any messages'
• click on 'OK'

What is retention policy on the imap 'Sent' folder.

• Right click on 'Sent' folder and select 'Properties'
• select 'Retention Policy' tab
• Usually, 'Use Account settings' is selected.

If your account setting - as mentioned above - are set to delete emails, then you need to :

• uncheck 'Use Account settings'
• select 'Don't delete any messages'
• click on 'OK'