Question: Mail links open new windows in Chrome instead of new tabs
Many have reported this issue over the past few months, at least through December. Here's the answer I found here that worked for me that was posted on November 15, 2017, before many of the questions: Shut down the Mac. Get ready to press command, option, R, and P at once. Click the power button and immediately hold all four of those keys down together for 15 seconds (or, if earlier, until you hear the second start-up sound). Then log back in, and your email hyperlinks should work again.
iMac, OS X El Capitan (10.11)