Office 365 Hosting: Deleted Emails

Everything working beautifully until the ISP pulled the email hosting, so thought going over to Office365 was a good idea.
Well, kind of, except in not realising a new EM Client account for the same email address was required, decided to continue with the old Basic account. Great with receiving, but the sending would not work for even after enabling smtp through Microsoft 365 admin center.
If EM Client had provided a warning/notification that the AUTH on the account has changed (possible?), it might have saved a considerable amount of time here.
After setting up the account (EM Client nicely detected it as Office365), the MS browser pop-up asked for 2FA on the mobile, and after downloading their Authenticator on the Android, discovered that nothing related to signing into the account worked on it. So 2FA not working, chose the Ask Later (14 day extension option).
All well and good, as things evolve, however, in synchronising to the server, all emails in there older than Monday were nixed. All except one - that was from an SSL security provider last May. The damage might already have been done in the synchronising the IMAP in the Basic account, perhaps MS are using an aggressive spam bot - who knows?
How odd, and annoying, as some of those emails were important. Perhaps, for next time, a prompt (or default setting) for auto-backup for an existing EM Client account when another is created for the same email address.
The fault can be portioned between me, Microsoft, and EM Client - although MS gets the lion’s share. Related: Office365 Broken, and Emails Won’t Send.