[exim] 'X=TLSv1: ...' in exim log file

Hi,

I've got Exim 4.54 running with TLS/SSL (and working better and better
thanks to help from you all on this mailing list), so I'd like to ask the
next question:

I've got 'X=TLSv1:RC4-MD5:128' in my log file for mail I send out (or at
least for the connection between my client and my server). I've noticed
'X=TLSv1:AES256-SHA:256' from mail received from a different mailing list.
My educated guess is that this is 256-bit encryption, and my Exim connection
is 128-bit.(?) What would I need to do to increase my Exim TLS/SSL
connections to 256-bit?

Thanks much!
Robert