SSRS not sending scheduled email to specific contact
This question shouldn't be posted here even if it is related to SSRS.

IMHO and according the comments it seems that the SMTP service used by SSRS is miss-configured.

Take a deep look at SPF record in your DNS to ensure it belongs to your domain. Some mail servers have an strong spam-filter activated to ensure all received mails are good and belonging to the mail sender domain. Among other spam-filters.

You can use some (online) tools to check your mail server configuration.