emClient Beta 10 cancels after database check

After sending an email, emClient stopped with an unexpected error.

Restarting it after sending a crash message leaded to a database check, summing up > 43k errors (!!!). Since then, emClient always checks db on start, always “fixing” > 43k errors, ending up in crash message.

I have a very bad feeling of having lost very much, very important mails since backing up version 9.

Any hints how to proceed?

Additional information: The sent message returns with „Undelivered Mail Returned to Sender« which seems to be sent again and again on start – maybe causing this effect.
How can I prevent emClient from sending it on startup?

I could start emClient with pressed STRG/CTRL button → offline mode (I changed my pc into flight mode to assure this).

Strangely enough, no (perceptible) database check took place in this mode.

To be on the safe side, I immediately created a database archive. After inspection, all e-mails are still there, and I have deleted all copies and returns of the undeliverable one. I don’t care whether this was necessary/useful: there must have been something wrong with it.

I could close emClient after this and restart it again normally without any unwanted side effects.