A degree of redundancy is built into the dns specification for dns MX records covering smtp services. A dns zone may specify multiple mx records with varying preference levels. The intent is that inbound email be directed to the first smtp server that is both available and preferred.
However, there are no similar redundancy mechanisms specified in the dns rfc standards for pop3, imap and outbound smtp services. There are also no standards for global load balancing of any email protocol.
But, there are standards compliant means to achieve full redundancy and global load balancing for these email services. Especially if the architecture is based on Microsoft Exchange Server with front end servers.
With edgedirector dns services, the administrator specifies multiple address records for each server name and optionally, specifies global coverage areas for these records. The dns service will return the appropriate dns records so that the correct server is used by clients. The server health monitoring system ensures that as long as one server is available, all clients will receive an appropriate ip address.