Emails in spam
Business E-Mail that lands in the recipient's spam folder, or never arrives at all, is one of the most pernicious problems a you can have because nothing looks wrong. The message leaves, and that is the last you hear of it.
Almost always the cause is technical and fixable: missing or incorrect SPF, DKIM and DMARC records, a shared server whose reputation somebody else ruined, or an address that has ended up on a blacklist.