Those email messages are INFO not errors. See Email Configuration for information about how to configure email.
The error about IndexWriterTask happens when it is indexing content into the search index. The content first gets serialized into the database then processed to write into the search index. It sounds like the content is too big for the field and the xml serialized content is getting truncated in the database and then it cannot parse the xml.
What database platform are you using?
You may need to delete the bad rows from mp_IndexingQueue
Hope it helps,