Are all the emails going from the same account, or is the failure related to a specific email account? Do the failed emails appear in your Sent folder, or are they perhaps still in Drafts?
If you can work out any common factor in what you’re doing that might influence the failures, that would really help. Unless you can be specific, it’s really hard for us internet randos to help, unfortunately.
Is the mac.com email address connected to Mailspring using the iCloud option or as a standard IMAP account? Perhaps try the other option if it’s not working properly?