A system comprising: a connection service configured to communicate with a plurality of client messaging application instances; a mailbox service layer configured to temporarily store and process message updates inbound from and outbound to one or more of the client messaging application instances, the message updates allowing synchronization between the client messaging application instances and one or more outside message service providers; a message service layer configured to communicate with the one or more outside message service providers and to translate message updates to a message format compatible with native rendering by a client messaging application instance, wherein one of the message updates comprises a message with an attachment, and characterized in that a translated message update corresponding to the one of the message updates includes an in-line reference for retrieval of the attachment that is stored on an attachment server, wherein the attachment of the one of the message updates is stored on the attachment server when the one of the message updates is translated to its corresponding translated message update; a first intermediary transfer layer communicatively coupled between the connection service and the mailbox service layer, the first intermediary transfer layer including a first inbound message data queue to transfer message updates from the plurality of client messaging application instances to the mailbox service layer and a first outbound message data queue to transfer message updates from the mailbox service layer to the connection service for communication to one or more of the client messaging application instances; and a second intermediary transfer layer communicatively coupled between the mailbox service layer and the message service layer, the second intermediary transfer layer including a second inbound message data queue to transfer translated message updates from the message service layer to the mailbox service layer and a second outbound message data queue to transfer message updates from the mailbox service layer to the message service layer; and, a conflict resolution module configured to resolve a message state consistency conflict between two or more message updates within one or both of the mailbox service layer and the message service layer, wherein the message updates originating from the message service layer have a higher priority over other message updates. a connection service configured to communicate with a plurality of client messaging application instances; a mailbox service layer configured to temporarily store and process message updates inbound from and outbound to one or more of the client messaging application instances, the message updates allowing synchronization between the client messaging application instances and one or more outside message service providers; a message service layer configured to communicate with the one or more outside message service providers and to translate message updates to a message format compatible with native rendering by a client messaging application instance, wherein one of the message updates comprises a message with an attachment, and characterized in that a translated message update corresponding to the one of the message updates includes an in-line reference for retrieval of the attachment that is stored on an attachment server, wherein the attachment of the one of the message updates is stored on the attachment server when the one of the message updates is translated to its corresponding translated message update; a first intermediary transfer layer communicatively coupled between the connection service and the mailbox service layer, the first intermediary transfer layer including a first inbound message data queue to transfer message updates from the plurality of client messaging application instances to the mailbox service layer and a first outbound message data queue to transfer message updates from the mailbox service layer to the connection service for communication to one or more of the client messaging application instances; and a second intermediary transfer layer communicatively coupled between the mailbox service layer and the message service layer, the second intermediary transfer layer including a second inbound message data queue to transfer translated message updates from the message service layer to the mailbox service layer and a second outbound message data queue to transfer message updates from the mailbox service layer to the message service layer; and, a conflict resolution module configured to resolve a message state consistency conflict between two or more message updates within one or both of the mailbox service layer and the message service layer, wherein the message updates originating from the message service layer have a higher priority over other message updates.