10.13.3 (17D39a) – Mail.app hangs while inserting recipients in a new e-mail window

Number:rdar://36668476 Date Originated:January 19 2018, 10:13 PM CET
Status:Open Resolved:No
Product:Mail Product Version:Mail.app 11.2 (3445.5.20)
Classification:Crash/Hang/Data Loss Reproducible:Always
Mail.app hangs while inserting recipients in a new e-mail window.
I'm attaching a screen recording while reproducing the issue, please note that the SBBOD is not visibile on the video but it started at about 2:50.
Macs that are experiencing this issue use three (3) Office 365 Enterprise accounts configured as Exchange accounts with Contacts enabled (please see the attached screenshot for an example). It seems that at a certain point Mail.app hangs while looking up names in the company's GAL (Global Address List) or in the accounts' contact lists. I've been able to reproduce the issue also with two (2) Exchange accounts with Contacts enabled and even with only one (1) account but, in this case, I had to quickly insert many more recipients to trigger the hang.
In the company I'm currently working for this issue is heavily undermining the feasibility to continue to use the native Mail app as the primary MUA on the top executive management's Macs.

Steps to Reproduce:
1. Compose a new e-mail and start inserting recipients picked up from the company's GAL (see attached video).

Expected Results:
Mail.app should not hang.

Actual Results:
Mail.app hangs.

macOS 10.13.3 (17D39a)
Mail.app 11.2 (3445.5.20)

All the Macs have Mail Act-On 4.1.3 [1] installed but I tried to uninstall it and Mail.app still hangs.

[1] https://smallcubed.com/mao/

Logs, screenshots and System Profile.zip
Mail hang.mov


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