Some providers, I suspect gmail here, have new requirements for e-mail. SPF entries, using DKIM to sign mails perhaps. I don't recommend actually following the

Some providers, I suspect gmail here, have new requirements for e-mail. SPF entries, using DKIM to sign mails perhaps. I don't recommend actually following these guidelines to the whims of Google although it might be good practice for some applications. But this is again not against spam, there are other interests involved here and not necessarily to the interest of the users and the mechanism can be used to make email exclusive overall. If it is just a spam filter, you might want to include some personal information although the filters have become far more aggressive because they had to. Scammers already use personalized info to send out fake mails. If you have a customer where this happened perhaps ask their IT what methods they are using. Perhaps adjusting the mail for a popular methods of spam filtering can solve the problem.