Message with Large Attachment Failed to get Sent

I tried to send a new message with two attachments (3MB and 5 MB).  eM Client kept trying to send but failed.  I tried to remove the message (so it wouldn’t keep re-sending) , but I couldn’t find the message in the Draft box nor the Outbox.  How could I fix this?

Hello,

Are you using an Exchange account?

-Dave

I get the same issue and get an error 507 message. I am sending from an Exchange account but it is not a server side issue because I can send the exact same message with the same attachments from OWA without any issues. Please advise what I can do to solve this issue.