Email notification problems

[Acorn]

Support Request Template
What version of phpBB are you using? phpBB 3.2.2
What is your board's URL? No answer given
Who do you host your board with? Shared hosting on Rackspace
How did you install your board? I used the download package from phpBB.com
What is the most recent action performed on your board? Update from a previous version of phpBB3
Is registration required to reproduce this issue? Yes
Do you have any MODs installed? No
Do you have any extensions installed? Yes
What version of phpBB3 did you update from? phpBB 3.1.10
What extensions do you have installed? Add User
Board Announcements
Large Font
Obscure Contact Us
Right Side Header Image
Stop Forum Spam
What styles do you currently have installed? My own customised prosilver
What language(s) is your board currently using? English
Which database type/version are you using? MySQL 5
What is your level of experience? New to PHP but not phpBB
What username can be used to view this issue? No answer given
What password can be used to view this issue? No answer given
What actions did you take (updating your board; installing a MOD, style or extension; etc.) prior to this problem becoming noticeable? This problem has clearly been going on for quite a while.
Please describe your problem. The error log is showing errors with notification emails and registration emails that are preventing them from being sent. This is a typical error log entry for a post notification that should have come to me:

Email error
?? EMAIL/SMTP
/forum/posting.php

Ran into problems sending email at Line 1175. Response: 550 This domain is not hosted here
.

<my correct email address> possibly an invalid email address?
Backtrace

Connecting to smtp.123-reg.co.uk:25
LINE: 1139 <- 220 smtp04.mailcore.me ESMTP Exim 4.89 - "Mailcore SMTP Service" Sun, 06 May 2018 02:44:18 +0100

# EHLO odbf.co.uk
LINE: 1486 <- 250-smtp04.mailcore.me Hello mail.odbf.co.uk [109.228.53.51]

LINE: 1486 <- 250-SIZE 83886080

LINE: 1486 <- 250-8BITMIME

LINE: 1486 <- 250-PIPELINING

LINE: 1486 <- 250-AUTH PLAIN LOGIN

LINE: 1486 <- 250 HELP

# MAIL FROM:<board email address>
LINE: 1155 <- 250 OK

# RCPT TO:<my correct email address>
LINE: 1175 <- 550 This domain is not hosted here

This is clearly happening for other users as well, with the same error message with their email address where mine is shown in the above error message.
Generated by SRT Generator

I had a similar error message when I tried sending a test email via the ACP. I then changed my email address to one from the same domain as the forum and tried again. This time I didn't get an entry in the error log (I suppose because the domain is hosted there), but the email was immediately returned to me board admin as undeliverable.

However, other people's email addresses are being marked as 'This domain is not hosted here', because, of course, they are not, so this is not a solution.

I am particularly concerned because we have admin activation of accounts and I suspect that notifications of account activation are not being sent out.

[Zoddo]

Try to set "Use SMTP" to No

[Brf]

<- 550 This domain is not hosted here

Generally this error means that the smtp server you are trying to use does not allow relaying.

[Acorn]

Thank you both. I changed the setting for smtp to 'no', and I actually had a notification this morning. And it wasn't returned to me as undeliverable either. It looks as though whatever is marking our emails as spam (gmail for one) is also happy with them now.