MySql include partition key in index

Why do you think anything but a scan should be used to pull back all the data? A full-text index won't really help - that helps you search those columns, but if you're just returning all the data (for any variety of WHERE clauses) then there's no shortcut to reading all of the data. Can I ask why a to_addr, which is presumably limited to ~320 characters by the SMTP standards (depending on which standard you believe), contains data > 4000 characters?

A lot of people think that a scan is bad. If you need to return a large amount of data, then often a clustered index scan will be used. Your where clause may lead to seeks being used to locate the rows to return, but a seek isn't going to work where the data in that column is that large. Are you just seeing a scan in the execution plan and assuming that must be the problem?

That's expected, most DDL operations on partitions will invalidate the indexes affected by the DDL. The ALTER TABLE docs state that on all relevant operations.

Specifically for truncate partition:

For each partition or subpartition truncated, Oracle Database also truncates corresponding local index partitions and subpartitions. If those index partitions or subpartitions are marked UNUSABLE, then the database truncates them and resets the UNUSABLE marker to VALID.

So local indexes are truncated and marked valid. Global indexes are invalidated unless you specifically request that they be maintained. (See further down.)
In your case, it looks like your primary key is not based on a local index – it appears in fact not to be partitioned, since you can't do an alter index ... rebuild on a partitioned index, you need to rebuild each partition. So the invalidation is expected.

To do that, you can add an UPDATE INDEXES / UPDATE GLOBAL INDEXES clause to your ALTER TABLE statement to have Oracle automatically maintain the indexes for you during the ALTER - See Updating Indexes Automatically. There are some limitations though, read the Considerations when Updating Indexes Automatically section carefully.

But that's not always desirable. Sometimes it's more efficient to rebuild the indexes after the load. In that case, your load process can often use the SKIP_UNUSABLE_INDEXES parameter (can be set at session level). (This parameter defaults to TRUE in 11gR2.)