Q: Responses to Emails get sent to different account.

Do you have any Rules set up?

Are these other accounts Aliases or true separate accounts?

In Mail is is set to send from one account always, or Last used account?

Well, I'm in 10.5.8 right now, but I think it's similar enough, check Mail>Preferences>Accounts>Account Info tab for outgoing Mail server in each, then Mail>Preferences>Composing>Send new mail from: for each.

Sorry, I haven't been able to figure out anything else that would do this, hopefully  we get more help in here.

Oh wait, Show the Long Headers in one of those Sent & see if there's a Reply to: different than the To:

Yikes, ...

Show the Long Headers in one of those Sent & see if there's a Reply to: different than the To:

I meant... Show the Long Headers in one of those Sent & see if there's a Reply to: different than the From:

In Mail>Preferences>Accounts>Outgoing Mail server, check & set use only this server...

I am having the exact same problem.  I or anyone of my friends sends an email to my Msm@gmail and its delivered to my Nis@gmail.  It just started and cannot find the reason or a solution to fix it.  I do receive emails in my msm@gmail from various institutions, i.e. Apple, Banks and official info from Google Addressed to my msm@gmail.

Anyone out there that can solve this ?  Beers on me.

sounds like a problem for Google

There are forums for all of their products

Gmail Help Forum - Google Product Forums

https://productforums.google.com/forum/#!forum/gmail

ADDED

If you get it solved, please remember to post the solution back to this thread

Check that you have assigned the correct SMTP server to each account.

Clearly you have misapplied the LDC SMTP server to the YDC account, change the SMTP server to the LDC account, then check all the others.