Missing mail questions

prawnballs

I posted some items from one character to another and the mail never arrived at its destination (they left my bag but nothing arrived other end). I have since discovered this is probably because the characters are on different shards.

My questions are:

Will the unreceived mails get returned and if so, how long to wait?

If they don't come back, when the supposed shards merge happens, will the mails go through then?

Thanks in advance!