Your mail server certificate is invalid error

Well, no such problem on Apple, cannot speak for Android, as I do not have Android.

The problem some off us had, was to do with the security that Microsoft had changed and updated and TalkTalk, the email provider had not updated, needed to be sorted, which now is, so may need to take it up with your email provider.

I AM referring to TalkTalk, and the original question posted by MarthaMerza specifically referred to Outlook for Android.

I had exactly the same issue - however, the gist of what I was saying is that even though the issue has been resolved (almost certainly not by Microsoft, although their actions initiated the problem) I still would not go back to using Outlook for Android for my TalkTalk email because it has so many other fundamental issues. Basically bug ridden.

Having said that, TalkTalk have proven themselves repeatedly to not be the "sharpest tools in the box" when it comes to security. For years they blithely ignored feedback about why their portal login page didn't even use HTTPS - until the crap hit the fan and they had that very large data breach.

Well, no such problem on Apple, cannot speak for Android, as I do not have Android.

The problem some off us had, was to do with the security that Microsoft had changed and updated and TalkTalk, the email provider had not updated, needed to be sorted, which now is, so may need to take it up with your email provider.

Personally I wouldn't thank them unless there is proof they have done anything. It could be that TalkTalk have amended their cipher set to "accommodate" this.

Besides Outlook for Android has SO many issues still- after tweaking the notifications sounds within Outlook for Andoid (I have four email accounts in it) notifications on at least one appear to have stopped working all together! Most of yesterday it kept repeating notifications for emails read HOURS ago. This is despite using the only two options available - Reset Account and removing it and re-adding it. Outlook 2016 (version 1806) on my main PC had no such issues.

Garbage app

Found the solution!

Regardless of what Microsoft says this is not about TLS 1.2 as Outlook still works with some exchange servers (e.g. Exchange 2010 running on Win2k8R2 with TLS 1.2 disabled).

This is about cipher suites supported by the server or any other proxy in between (e.g. ISA).

You have to enable TLS RSA with AES 256 or 128 cipher suite support to get email working again.

In case your Exchange server is running on Win2k3 or you have other infrasteucture in between on it - get this patch installed:

https://support.microsoft.com/en-us/help/948963/an-update-is-available-to-add-support-for-the-tls-rsa-with-aes-128-cbc

I wouldn't bother waiting.

Bin Outlook and switch to one of the many (free) alternatives which show Outlook to be the utter dross it is.

I tried BlueMail, as others have suggested, and it is SOOOO much better.