Glad you got sorted! I don’t think I could connect with OWA365! Liking your ‘Wot no…?’ avatar - you need to add ‘32-bits’. 
Nah, Should be “Wot no Windows”
I was thinking of Chad, from my early days in computing. Found some examples then decided to create my own using paint app this afternoon.
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Cool! 
Just a small supplementary question:
Why doesn’t the normal Thunderbird (blue bird) icon appear on desktop or in Startup/Internet app list?
I have always had this generic email icon.
How did you install Thunderbird? Did it come pre-installed, was it Flatpack, Snap or did you install from terminal / package manager?
From (Zorin) Software, but I don’t have any record of installing it, so was it part of 15Core maybe.
I remember Firefox was included as part of Core package.
I have just found this:
http://forums.mozillazine.org/viewtopic.php?f=39&t=509891
I may have a tinker, but is not critical.
No it’s not part of 15 Core. Zorin ships with Evolution. 
The mozillazine link I posted above is from 2007 so not exactly current, but I have found the Thunderbird icons parked here:
/usr/lib/thunderbird/chrome/icons/default
I don’t know where the icon I have is being picked up from or where the proper TB icons should be placed to be used.
@swarfendor437 confirmed Evolution is mail client in the Core package.
Although I removed Evolution when I installed Thunderbird (via Software), I still have two Evolution folders:
/usr/lib/evolution
/usr/lib/evolution-data-server
Also synaptic indicates these Evolution left-overs from the software removal.
Is it safe to remove these two items using synaptic, or do they serve a purpose for the Z15Core package? (I need all the root space I can get)
.
PS: If mods want to split this thread (as original Calendar problem is solved and now different topic) then feel free e.g. “Thunderbird wrong icons”
I would see what comes back when you try to remove those packages in a terminal. In earlier versions of Gnome, Evolution calendar was embedded in the system - that’s how Gnome/Ubuntu got it’s date function in the clock! Try to remove Evolution Calendar and your system would disappear! Perhaps this is one for AZorin to answer?
Have you seen the security threat from Tesla Malware on Thunderbird? I posted it in Security News thread.
@AZorin . Yes be good for us to know if parts of Evolution are critical to ZorinOS.
Any ideas about where Thunderbird icon(s) should be located for them to work as shortcuts etc?
There are so many items of malware that can be introduced via email, so have to be wary all the time, particularly in Windows environment.
usr | share | applications | Thunderbird
You can copy and paste to your desktop (well I can in Cinnamon DE which is what I have just done to check - not sure about Gnome.
Yes, the evolution-data-server is used by the Online Accounts feature which is a part of the desktop. Despite sharing a name with the Evolution app, it acts more like a library and can function independently of the mail/groupware app.
It's not recommended to remove the evolution-data-server packages from the system in order to keep this functionality and maintain system stability, as it's a dependency of the desktop.
@AZorin . Thanks for your answer, much appreciated. I am just glad that I sought advice before removing those two “Evolution” items and so avoided a breakage.
@swarfendor437
Well that location has the generic mail client icon showing against Thunderbird, so explains why that is on my desktop and not the bird.
The proper icons at:
/usr/lib/thunderbird/chrome/icons/default
are just different size .png image files of the bird.
I give up. I thought it was going to be easy to swap the icon, but seems not the case.
On my build, the thunderbird icon used is whichever icon set I am using. I am using Zorin-ish… So, it should really be the same thunderbird as you.
IF that is the case, then it should be easy to provide an icon for Thunderbird- As I make Icon Sets all the time, I do it with each set I make.
Mine has always been the default icon - apart from where somehow I appear to have two of them (have I been hacked?) in the menu, plus I have two waterfox icons - very strange stuff!
@swarfendor437. Are you are talking about duplicates at usr/share/applications ? If so, you are not alone. A small number of applications are listed twice and smaller number 3 times. I don’t know if that is a bad thing or not.
@Aravisian. I am not overly concerned that I do not have the normal blue bird icon for my Thunderbird menu or desktop shortcuts. I thought it may be possible to simply pick an alternative icon from a list, but seems not the case. It is not worth expending more effort to fix, especially as may be corrected if I have to reinstall Thunderbird for Z16 
More:
Aravisian got me thinking about themes. I use simple Zorin95 icons. However, if I switch to Gnome (or anything other than Zorin95) then I get the blue bird icon for Thunderbird. So the problem here seems to be associated with Zorin95 icons. I will not change to one of the other icon sets just for a blue bird, but strange that the Zorin95 set is only one that is different.
That’s because Zorin-95 uses ‘paper’ (flat) icons. I alway prefer hicolor - which is probably why I get the mythological Thunderbird icon.
An icon theme can include an "inherit" list. This list accounts for icons that may be missing in the set by allowing the system to fallback to a different set to 'inherit' the missing icons from that set, rather than leave a black imageless gap.