Maybe the same: I recive emails from GMail but cannot send any. KMail/Akonadi is "less secure" aplication for Google. Even when I accept this kind of connection's on GMail side, nothing has changed. 
I think that this problem is not only KMail/Akonadi related, because Trojita cannot connect with GMail at all.

Had the same after updating to latest version on Fedora 28.
kontact-18.04.3-1.fc28.x86_64
kdepim-runtime-18.04.3-1.fc28.x86_64
kdepimlibs-4.14.10-26.fc28.x86_64
kdepim-runtime-libs-18.04.3-1.fc28.x86_64
kdepim-apps-libs-18.04.3-1.fc28.x86_64
kf5-libkdepim-akonadi-18.04.3-1.fc28.x86_64

The problem went away after enabling "less secure" application via the gmail web interface.

Also, at #fedora-kde on freenode (IRC), it was said that the "less secure" option is needed until support for oauth(?) is made available - it should be in the next version.

Unfortunetely "less secure" option doesn't work for me. Tested on KMail, Trojita and Kube. On KMail I can receive mails, but on Trojita and Kube there're not possible to connect to GMail's server at all.

Today I changed "less secure" option on GMail settings one more time, and one more time accept "unknown access" to my account (yes, it was me), and everything works well, again.
So, probably this "bug" is GMail's related.

(In reply to MartinG from comment #2)
> Had the same after updating to latest version on Fedora 28.
> kontact-18.04.3-1.fc28.x86_64
> kdepim-runtime-18.04.3-1.fc28.x86_64
> kdepimlibs-4.14.10-26.fc28.x86_64
> kdepim-runtime-libs-18.04.3-1.fc28.x86_64
> kdepim-apps-libs-18.04.3-1.fc28.x86_64
> kf5-libkdepim-akonadi-18.04.3-1.fc28.x86_64
> 
> The problem went away after enabling "less secure" application via the gmail
> web interface.
> 
> Also, at #fedora-kde on freenode (IRC), it was said that the "less secure"
> option is needed until support for oauth(?) is made available - it should be
> in the next version.

Mine was already set to "Allow less secure apps: ON" and dated 3 May when it was given access so what did the Dialog do yesterday when it popped up?  I'll try removing the access and recreating it and see what happens

(In reply to Pawel from comment #4)
> Today I changed "less secure" option on GMail settings one more time, and
> one more time accept "unknown access" to my account (yes, it was me), and
> everything works well, again.
> So, probably this "bug" is GMail's related.
Mine was already set to "Allow less secure apps: ON", I'll try removing the access and recreating it and see what happens

(In reply to Pawel from comment #4)
> Today I changed "less secure" option on GMail settings one more time, and
> one more time accept "unknown access" to my account (yes, it was me), and
> everything works well, again.
> So, probably this "bug" is GMail's related.

I've found a solution.  I changed the "Allow less secure apps: ON" to OFF on my Google settings for webmail and that removed Akonadi from the permitted apps but that didn't do anything to fix it though so this might not need to be done. 
I then went through the kmail settings for "Receiving" emails on the google account, on that page is said Authentication is "Gmail" -  i changed this setting to "PLAIN" and now it all works.  
I've not had the Akonadi/Google dialog again as yet to reestablish "permissions" to read/send emails via google.

> I then went through the kmail settings for "Receiving" emails on the google
> account, on that page is said Authentication is "Gmail" -  i changed this
> setting to "PLAIN" and now it all works.  

Seconded. The problem happens after a Tumbleweed update to Akonadi, with gmail authentication it doedsn't work, with PLAIN it does.

possibly related. 
kmail 18.08.0 refuses to connect to my IMAP accounts no matter the login settings. Also I'm behind an internal proxy with exceptions for the imap server. I tried it with it enabled and disabled and with proxy set to automatic, manual and off in kde settings.
The settings are correct though as Trojita connects without problems

akonadiconsole outputs this:
0x55c36ab58c60 { "error": false, "generation": 1535840819, "message": "Not Really IMAP server", "protocolVersion": 60, "response": true, "serverName": "Akonadi", "tag": 0, "type": "Hello" } 
0x55c36ab58c60 { "response": false, "sessionId": "akonadi_imap_resource_0", "tag": 2, "type": "Login" } 
akonadi_imap_resource_0 (0x55c36ab58c60) { "error": false, "response": true, "tag": 2, "type": "Login" } 
akonadi_imap_resource_0 (0x55c36ab58c60) { "resourceId": "akonadi_imap_resource_0", "response": false, "tag": 3, "type": "SelectResource" } 
akonadi_imap_resource_0 (0x55c36ab58c60) { "error": false, "response": true, "tag": 3, "type": "SelectResource" } 
akonadi_imap_resource_0 (0x55c36ab58c60) { "resourceId": "akonadi_imap_resource_0", "response": false, "tag": 4, "type": "SelectResource" } 
akonadi_imap_resource_0 (0x55c36ab58c60) { "error": false, "response": true, "tag": 4, "type": "SelectResource" }

Hello,

I can receive emails from Gmail (pop3, ssl / tls 995) but not send (smtp ssl / tls 465) I tested several solutions (port etc ...) without success.

This is unfortunately not the first time this year.

If Akonadi can not be corrected for good then maybe the Kde-Pim team should stop trying to keep it alive: persisting is commendable, but persevering in this kind of mistake is stupid.

I think I will finish with kontact and kde at the same time and go to take refuge at Gnome and thunderbird; I like kde but I'm not stupid to the point of depriving me of a feature as basic as sending mail ...

(In reply to Olivier BELLEUX from comment #10)
> Hello,
> 
> I can receive emails from Gmail (pop3, ssl / tls 995) but not send (smtp ssl
> / tls 465) I tested several solutions (port etc ...) without success.
> 
> This is unfortunately not the first time this year.
> 
> If Akonadi can not be corrected for good then maybe the Kde-Pim team should
> stop trying to keep it alive: persisting is commendable, but persevering in
> this kind of mistake is stupid.
> 
> I think I will finish with kontact and kde at the same time and go to take
> refuge at Gnome and thunderbird; I like kde but I'm not stupid to the point
> of depriving me of a feature as basic as sending mail ...

I use imap with google and if i use auto-detect it configures as STARTTLS, port 587, PLAIN authentication and it sends ok.