The channel settings restrict who can send email, such as the channel moderation setting being turned on. Check with a team owner or a channel moderator to see if you have permission to send email to the channel.
The email contains more than 50 inline images.
The email has more than 20 file attachments.
The email has an attachment that's more than 10 MB.
The email includes the channel email address as part of a distribution list. Make sure that the channel email address is being used directly.
The SharePoint folder associated with the channel was deleted, renamed, or hasn't been synced. To revert a renamed folder or sync a folder: