Microsoft Sharepoint fails to send emails to recipients when Web Protection is enabled within Sophos Anti-Virus.
First seen in
Sophos Anti-Virus version 10.0.*
Microsoft Sharepoint is unable to correctly send emails if any third party LSP is stacked on top of the UDP Provider. Sophos -Anti-Virus stacks its LSP on both the UDP and TCP Providers in accordance with Microsofts guidance in the LSP SDK for intercepting traffic from Internet Explorer.
What To Do
It is recommended that you follow this Microsoft knowledgebase article.