There should be "are" after "but".
Student or Learner
Are these sentences correct?
We received your email yesterday but unable to open the attachments. As there are a lot of documents, could you split them up and send to us by two or three e-mails.
Or use 'failed' instead of 'unable' after 'but'.
I am not a teacher.
"Could not" would probably work better there, because "fail" has a somewhat different meaning.
Not a teacher here. Just an English buff.
"... and send them to us over/in two or three emails."
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.