This was an incident happened to one of my colleague and I was part of troubleshooting team.
He was complaining that his domain account getting locked daily. Their domain has security configuration,
1. Password is expiring quarterly, every three months.
2. Three continues invalid attempts to login will lock the domain account.
Since I had the same issue before, I thought it might be a service. Once I have configured SQL Server Express service account to login from my credential. Though I am not using this Express instance it has locked my domain account.
So I checked all the services but it went in vain.
We thought it is something to do with Outlook settings, which we reinstalled again didn’t work.
Then we stopped guessing and thought to approaching it more planned way and we started to examine the Security Log. (You would asks, “Why the hell you didn’t do that before” 
Well, we found a clue there and it says logon failures for database mail.
Then examine the database mail configuration, yeap! we got it. SMTP authentication was set with Basic authentication instead he could have done with Windows Authentication.
After configuring database mail later his password has expired but since he has used basic configuration, still database mail was trying with the previous password. In his database server, there is job to send notification using database mail. with this job database mail getting executed and with that domain account is getting locked.
