Mail keeps saving the same drafts

Hello

i tried to send a large attachment(5 gigs) with Mail,

to see how Mail will handle it.

I attached the file and send it like a normal email.

It started sending the email, saw it in outbox, and left it to do its thing.

When i got back i could not seem to find the email either in the outbox nor the send folders.

Since then every time i open Mail in the left lower corner of the app window

i get this witch never seems to complete and makes my mac run slow.

Send large attachments with Mail drop check box is checked in Mail's preferences.

I am running High sierra 10.13.2 on a 3,1 i7 iMac.

Any suggestions?

Thank you in advance!