No emails coming from WordPress

Hi guys,
I built a site for a client and when I went to change the admin email to their email address, it didn’t send a verification email.

So, I changed the admin email manually from phpmyadmin to their email address, at which point the contact form (ninja contact forms) stopped sending confirmation/submission emails at all, even if I manually changed the WP email back to mine.

Now, the site doesn’t send any emails at all.
I’ve tested with the ‘Check Email’ plugin and never received the test email, and I have also tried to set up an SMTP option through WP Mail SMTP, and get the following error:

Versions:
WordPress: 4.9.5
WordPress MS: No
PHP: 5.6.22
WP Mail SMTP: 1.2.5

Params:
Mailer: smtp
Constants: No
ErrorInfo: 
Host: localhost
Port: 25
SMTPSecure: string(0) ""
SMTPAutoTLS: bool(true)
SMTPAuth: bool(false)

Server:
OpenSSL: Yes
SMTP Debug:
[empty]

If I try to change the admin email settings from WP, it says it’s pending the change and has sent a verification email to the nominated email, but that email never arrives.

Any thoughts?