WPMailSMTP stopped working

Hi,
our contact form created with WPForms suddenly stopped working, mail tests from WPMailSMTP fail with the following error:

Versions:
WordPress: 5.2.1
WordPress MS: No
PHP: 7.0.33
WP Mail SMTP: 1.4.2

Params:
Mailer: smtp
Constants: Yes
ErrorInfo: SMTP connect() failed. https://github.com/PHPMailer/PHPMailer/wiki/Troubleshooting
Host: smtp.office365.com
Port: 587
SMTPSecure: tls
SMTPAutoTLS: bool(true)
SMTPAuth: bool(true)

Server:
OpenSSL: OpenSSL 1.0.2r 26 Feb 2019
SMTP Debug:
2019-06-12 08:13:49 Connection: opening to smtp.office365.com:587, timeout=300, options=array (
)
2019-06-12 08:13:49 Connection: opened
2019-06-12 08:13:49 SERVER -> CLIENT: 220-box5654.bluehost.com ESMTP Exim 4.92 #2 Wed, 12 Jun 2019 03:13:49 -0500
220-We do not authorize the use of this system to transport unsolicited,
220 and/or bulk e-mail.
2019-06-12 08:13:49 CLIENT -> SERVER: EHLO http://www.mhrsconsulting.uk
2019-06-12 08:13:49 SERVER -> CLIENT: 250-box5654.bluehost.com Hello http://www.mhrsconsulting.uk [162.241.219.161]
250-SIZE 52428800
250-8BITMIME
250-PIPELINING
250-AUTH PLAIN LOGIN
250-STARTTLS
250 HELP
2019-06-12 08:13:49 CLIENT -> SERVER: STARTTLS
2019-06-12 08:13:49 SERVER -> CLIENT: 220 TLS go ahead
2019-06-12 08:13:49 SMTP Error: Could not connect to SMTP host.
2019-06-12 08:13:49 CLIENT -> SERVER: QUIT
2019-06-12 08:13:49 SERVER -> CLIENT: X`�M�B���������r�zS�r221 box5654.bluehost.com closing connection
2019-06-12 08:13:49 SMTP ERROR: QUIT command failed: X`�M�B���������r�zS�r221 box5654.bluehost.com closing connection
2019-06-12 08:13:49 Connection: closed
2019-06-12 08:13:49 SMTP connect() failed. https://github.com/PHPMailer/PHPMailer/wiki/Troubleshooting

We are using an Office365 account to send emails and the same account works perfectly in Outlook.

Can someone please point me in the right direction with this? I have already spoken to our web hosting provider (Bluehost) but they are adamant that this is not a problem at their end.

thank you,

Rob