Thread:Extension talk:ConfirmAccount/Problem with emails after unistalling ConfirmAccount/reply (3)

Well, it's possible the server you are using is blocked; maybe SpamHaus can tell you if your IP had problems in the past. There are many websites around the web which ask users to only register with gmail because all the others block their emails.

So, yes, using Manual:$wgSMTP to configure a "trusted" email sender will hopefully help, though I can't be sure.