I have a problem with my desktop Manager software, in the email settings. I have configured email correctly and I am able to send test mails. But when I send an invoice, quote and every other file, my clients do not receive the invoice. I do not also receive a copy.
If you are using custom SMTP, who knows… maybe SMTP server you are using is misconfigured. It seems like an issue is outside Manager.
What happens when you disable custom SMTP, do emails work? If yes, then issue is certainly with your custom SMTP server.
Test those settings you are using in Manager in another mail program and see whether other program has the same issue. If it has, then it’s SMTP server issue you are using.