What are the sp_send_dbmail return values? I have an email failing with function return value of '101' and @@error value of '0' (no rows are added to sysmail_allitems).

Where can I find documentation on return codes for this function?

Example code showing how I obtain the above values:

exec @result = msdb.dbo.sp_send_dbmail
    @profile_name = 'OBFUSCATED',  
    @recipients = @DL,  
    @subject = 'OBFUSCATED',
    @body = @emailBody,
    @body_format='html',
    @query = @reportQuery,
    @exclude_query_output = 1,
    @attach_query_result_as_file = 1,
    @query_attachment_filename = @filename,
    @query_result_separator = @temp,
    @query_result_header = 1,
    @mailitem_id = @mailitem_id
    ;

    set @temp = @@ERROR;
Error code 101 is because your query result attachment is larger than the &quot;MaxFileSize&quot; Database Mail configuration option.
Investigation
My first attempt at figuring this out was to run sp_helpttext on the sp_send_dbmail procedure to look at the implementation:
EXEC sp_helptext 'sp_send_dbmail';

The main body of that procedure returns a bunch of different status codes, from 0 up to 21.  It doesn't mention 101.  However, it calls into a few other stored procedures, like sysmail_verify_profile_sp, so I glanced in that one:
EXEC sp_helptext 'sysmail_verify_profile_sp';

But that only returns codes from 0 to 4.
Later the proc calls into sysmail_verify_addressparams_sp, let's check it out:
EXEC sp_helptext 'sysmail_verify_addressparams_sp';

Shucks, that only returns 0 or 1.
Next up is sp_RunMailQuery:
EXEC sp_helptext 'sp_RunMailQuery';

It calls sysmail_help_configure_value_sp and sp_isprohibited, both of which only return 0 or 1.
Black Box
At the end of sp_RunMailQuery, a system extended stored procedure is called: xp_sysmail_format_query.
We can't grab the source code for this, but I had a hunch from earlier in the proc:
--Get the maximum file size allowed for attachments from sysmailconfig.  
EXEC msdb.dbo.sysmail_help_configure_value_sp @parameter_name = N'MaxFileSize',   
                                            @parameter_value = @fileSizeStr OUTPUT

I noticed that you are using the attachment parameter, and the query results + max file size both get passed into the extended stored procedure as well.
Repro City
I set my Database Mail maximum file size to 10 bytes, and then ran:
DECLARE @result AS INT;

exec @result = msdb.dbo.sp_send_dbmail
    @profile_name = 'DBMail',  
    @recipients = 'myemailaddress@something.com',  
    @subject = 'This is the subject',
    @body = 'This is the body',
    @body_format='html',
    @query = 'SELECT * FROM sys.messages;',
    @exclude_query_output = 1,
    @attach_query_result_as_file = 1,
    @query_attachment_filename = 'big-file.csv',
    @query_result_separator = '===',
    @query_result_header = 1;

SELECT @result;

Note: the &quot;sys.messages&quot; table has a bunch of string data that I knew would be more than 10 bytes.
And sure enough, I got 101 as the return code:

Bugz
As CR241 helpfully pointed out, the documentation says that @@ERROR should contain the error id number that corresponds to the appropriate error message in the sys.messages table:

The error code for the statement that failed is stored in the @@ERROR variable. - sp_send_dbmail - Return Code Values

Instead it's 0 in my testing (which you noted in your original question).  This seems like a bug to me.