Yes you will still get that message due to Gmail tried and failed. Sometimes this message can also come from the eg: Gmail “Drafts” folder , Gmail “Trash” folder or even the Local “Outbox” folder at the bottom. So if there is nothing in Gmail “All Mail” folder, check those other folders. When you find it, delete it and clear that email from the trash and then close and reopen EMC. Obviously copy the email first to a new email.
Note:- If you still cannot find it anywhere, go online to Gmail and search for the message as eM Client if you use IMAP merely mirrors Gmail online.
Once you find / fix the problem , if you then want to send file attachments over 25MB in size via Gmail check out this thread for how to link a file attachment via Google Drive to eM Client. How to attach files (as a link) in Google Drive to eM Client - Frequently Asked Questions - eM Client
