is it possible to resume a transfer at an other moment?
if so, how?

It is not possible. One of the reasons is that if you cancel a transfer mIRC deltes the file, and if the download somehow breakes the file is saved to the place it stopped without contents for resuming because mIRC doesn't have such an option.

That didnt quite make sense to me, however.

The answer is Yes, and No.

If you cancel the download, then No you cannot resume it.

If the download fails, through no fault of your own, mIRC can attempt to resume it, if you goto, ALT + O - DCC - 'If file exists' You will see an option there in a dropdown box, 'Resume', though it might be worth mentioning, you should NOT rely on this method of be successful at all times as it is not guarrenteed to resume.

Eamonn.

I've done that many times and it's resumed just fine every time.
I don't think it matters why the transfer failed, if the file matches,
it will resume, assuming no other problems exist.

Edit: I've even resumed from a different source after cancelling.

You are right, somewhat confused myself there, thanks

Eamonn.