Version 8.0-20160426
I'm having a problem where upon clicking a "Send email" button in Odoo, in that the mail that is being sent shows the circle "waiting" logo and takes minutes to send leaving the user unable to work further in Odoo until the mail has been sent.

This only seems to happen in about one in three emails, but it's significant enough to be a problem for my users.

A little background 

I have Odoo hooked up to my Office 365 SMTP.

Following some of the advice given in the following thread, the steps below are how I have configured Odoo to use Office 365's SMTP relay: https://www.odoo.com/forum/help-1/question/outgoing-mail-server-not-working-with-smtp-office365-com-35676
Supplemented by using Option 3 from the Office 365 guide here:
https://support.office.com/en-us/article/How-to-set-up-a-multifunction-device-or-application-to-send-email-using-Office-365-69f58e99-c550-4274-ad18-c805d654b4c4

1. On Office 365 >admin>domain>click primary>Exchange Online>
   Copy the mx domain address (which should look something like: mydomain_com.mail.protection.outlook.com)
   This is the name of my outgoing SMTP server for Odoo - to be pasted later

2. Copy the recommended SPF rule and add in my Odoo server's IP, then add SPF record to my DNS records
    v=spf1 ip4:10.10.10.10 include:spf.protection.out...

3. Back to admin on Office 365 >admin>exchange>mail flow>connector>add
   Add a new rule From My organization's email server to office 365
   Add the Odoo server's ip address

4. On Odoo Settings>Config>General settings:
   Uncheck "Use external authentication providers..."

5. On Odoo Settings>Technical>Outgoing Mail Servers
   Smtp server: mx endpoint of domain which copied on step1.
   Port: 25
   Security: TSL(STARTTLS)
   No username or password required

6. In Odoo go to Settings>Technical>System Parameters - Then remove mail.catchall.domain and mail.catchall.alias 

My messages are all being sent - there are no issues there, but as I say,  Odoo seems to regularly hang when sending messages and it's driving my user's insane.  

I've been over the settings time and time again to ensure everything is set up correctly, but I can't see any reason why this might be happening. I'm hoping someone here might be able to shed some light on the issue!