I am sorry to hear that this is not a problem with Thunder Bird this is most likely a problem with your email service please contact them and mark this ticket as solved thank you for choosing Thunder Bird. the email for support for this company is [email protected]

Given the poor retention policy that BestWeb has ( They only guarantee a couple of days) http://www.bestweb.net/assistance/spamfilter/message-trash.shtml You may well be better using a POP account and simply downloading all email to Thunderbird from the inbox and maintaining only that local copy. Not all that convenient if you want to use your phone to read mail, but otherwise it may better suit your use case.

I don't actually store the emails in the "Trash" folder. It's in a "Deleted Items" folder. I will check with Bestweb but I've been using them as my ISP for 18 years and this is something that has never happened before.

Spoke with my ISP. They are not deleting my emails. They helped me sort out that my Iphone is responsible. There's a setting that deletes them after 7 days. My Iphone was reset last week and I did not realize it I needed to change that parameter. Bad Iphone!!  :-)