Question about forwarding email (not specifically SA, pointers greatly appreciated)

[Thomas Cameron via users]

Howdy, all -

This is not strictly SpamAssassin related, but y'all probably know where 
to point me to make this work.

I built email servers for a non-profit I volunteer for. If email comes 
into the server for presid...@myassociation.org, I would normally just 
create an alias in /etc/aliases so that emails to president@ get 
forwarded to the president's "real" email address, say 
presidents_real_em...@gmail.com.

The problem is, when I send email to presid...@myassociation.org, gmail 
rejects the forwarded email because it appears to come from my personal 
domain, not the mythical myassociation.org domain. DKIM, DMARC, and SPF 
all fail, which I totally understand.

How can I make this work? Is there a good way to use something like 
/etc/aliases to forward emails to the domain I manage to another 
recipient? Or is there something better I can do?

I apologize this isn't strictly SA related, I am just hoping someone can 
give me advice or provide I link to follow on how to make this work.

Thanks,
Thomas