[exim] Exim authentication as client and $sender_address

I use the following client authenticator to connect my Exim in smart
relay to my provider's SMTP server and send email:

fixed_plain_client:
     driver = plaintext
     public_name = PLAIN
     client_send = ^$sender_address^${lookup{$sender_address}\
         lsearch{/etc/exim/exim-client.passwd}{$value}{fail}}

It is all working fine, but there are two things I would need some
clarification about:

1. Does Exim authenticate as a client and send one message at a time by
default (and authenticate again for next message to be sent) - or does
it try to send several messages on the same authentication session?

2. If it tries to send several messages in one go after authenticating -
what does $sender_address refer to? Is it the sender address of the
first message which it tries to send?