Update configure-mail-server-exim4.md

Exim>=4.94 transport doens’t allow taintend parameters in commands (3 years ago), so if you use current  SYMPA configuration for exim4 you would have this error 

"sympa-request@xxxx R=sympa_queue_aliases T=sympa_queue_transport: Tainted arg 1 for sympa_queue_transport transport command: 'sympa-request@xxxxxxxx’  

Upgrade local_part and domain variables to local_part_data and domain_data.

More information

https://www.gentoo.org/support/news-items/2021-05-04-exim-transports-disallow-tainted.html