Error message due to spam when sending emails

When I want to send mails from my account, I always get a spam warning since today.

Sending via other mail accounts and other programs, for example Outlook for Windows or via webmail from Vodafone, works perfectly.

Why is that?

The content of the error notification (“spam id=…”) is the response of your email provider when the app tries to send the message. Please contact them to find out why they’re classifying your message(s) as spam.