My understanding is that Microsoft said "peace out" to EAS support on Outlook. You may be better off switching it to MAPI.

While Outlook may work with EAS, Microsoft isnt supporting it, which makes it harder for SmarterTools to support it.

EAS worked perfectly before i made the update. As long SmarterTools sells EAS to us, we can expect, that EAS is working good with Outlook 2016.

EAS is a mobile sync protocol. So while you do purchase EAS, the EAS protocol is designed for mobile devices. So while SmarterTools do sell it, if the company that designs the email clients that uses it drops support for it, there is nothing they can do.

Important Information for Outlook 2016 and EAS Users (smartertools.com) 

Your best bet is to use IMAP (probably not best with Outlook) or MAPI.

I would only switch to MAPI if your users need the functionality of MAPI. Otherwise, IMAP and EAS over/out perform MAPI by far - with SmarterMail. I am still running EAS in Outlook 2016 [on one workstation] with current version of SmarterMail and it is way faster than MAPI. It also does not give me sync problems with Contacts. That is still a thorn in my side with MAPI protocol. I have had to delete .ost files for customers [and myself] to resync soo many times, I've lost count.

@Seph : Microsoft has dropped EAS support in Outlook and so even if you want to keep using it, you simply CANNOT.

As written here (please read it all!): https://www.smartertools.com/blog/2017/10/04-Outlook-2016-and-EAS

It is not possible to use EAS reliably if Microsoft removes it from Outlook.

Only with older versions of Outlook is this still possible, but it is no longer guaranteed that it will work well, and this is the will of Microsoft. Nobody can do anything about it.

You have to realize that you have to surrender to this evidence.

If you don't trust us or SamrterTools, then read it directly from Microsoft's site: https://support.microsoft.com/kb/2859522

Since SmarterMail is seen by Outlook as if it were an Exchange server, this is not supported.

You can try to change the way Outlook configures accounts by modifying the Windows registry parameters (as mentioned in other posts), but this isn't supported by Microsoft, so it's not guaranteed to work.

And remember this: it is not a problem caused by SmarterTools, it is Microsoft that has chosen this path ...