If you are having issues sending email from your desktop/laptop/tablet email client e.g. Outlook, Mail etc please first try accessing the webmail service and sending an email from there.
To access the webmail service go here https://webmail.yourdomain.com (where yourdomain.com is your domain name)
The username is your email address and the password is the same password you use for that email address.
If you are able to send from the webmail service and your email recipient receives the email without issue, then this could indicate you have an issue with your local settings for whatever reason or perhaps the IP address you are sending from has been blocked.
From the location where you are unable to send email e.g. your home or office, visit this website https://whatismyipaddress.com/ where your IP address will be shown to you. Please create a support ticket and provides us with that IP address and the email account(s) you are having problems with.
If sending an email via the webmail service is NOT getting to your recipients either then this would suggest there is an issue at our end with your email account. Please raise a support ticket for us to investigate further.