trustd process runs away until Apple Mail quit

I recently received a new work MacBook Pro running Sierra. In the last week or so, I have noticed my fans kicking into high speed. When I look at Activity Monitor, it shows the 'trustd' process using between ~175% and 350% of my CPU. In my testing, the one thing I have found that will get it to stop is to quit Apple Mail.

I can restart Mail and it may be fine the rest of the day. However, today it has happened 4 times. I installed Outlook to see if it had a similar affect but, so far, trustd has not runaway when Outlook is the only mail app running (but this has been a short test so far).

From my investigation, trustd is pretty new, arriving with Sierra. It is related to verifying certificates, but beyond that, I don't know a lot. 😊

In Activity Monitor, I do see 4 occurrences of 'trustd'. Their respective users are 'root', '_spotlight', our IT dept, and my user account. the only one that runs away is the one associated with my user account.

Any ideas on why, all of a sudden, trustd starts consuming so much of my CPU?

Running 10.12.6 and Apple Mail 10.3 on a 2017 MacBook Pro 15" touchbar with 16GB RAM

Regards,