Today I had time to carry out further tests and it SEEMS that the disappearance of the emails has a dependency: depending on which is the starting mail system of the emails.
In my case I tested from MS Exchange OVH hosted, Libero Mail - an Italian provider -, OUTLOOK.COM (all of these via my Outlook on my PC).
I also tried the GMAIL webmail and the webmail of my SmarterMail server.
Well, I discovered this:
- e-mails sent with the OUTLOOK.COM account, if they are moved, practically always disappear
- e-mails sent with the LIBERO MAIL account, if they are moved, disappear often, but not always
- e-mails sent via the Smarter Mail server, if they are moved, NEVER disappear!
- if the e-mails sent via the GMAIL webmail are moved they do NOT seem to disappear
- e-mails sent via OVH hosted Exchange if they are moved do not seem to disappear
So the problem needs to be investigated further and you have to do more tests because it seems that SmarterMail behaves differently depending on the origin of the emails.
Probably some providers do something inside the headers or the content of the mails that SmarterMail DOES NOT LIKE and that therefore triggers the disappearance of the mail when moving from one account to another.