Outgoing Mail Has Been Suspended

Symptoms

When attempting to send mail from the webmail client Roundcube, you receive an error that indicates the email account is suspended:

SMTP Error (550): Failed to add recipient 'user@recipient.tld' (Outgoing mail from "user@yourdomain.tld‹" has been suspended.).

Description

This error will occur if the cPanel user or email account you are using has been suspended specifically from sending mail. You can manage this setting from within the cPanel account.

Workaround

If a single account is affected:

1. Log into the server as the root WHM user and access the cPanel account, or log in directly to the cPanel account as the user. 
2. Access "Email / Email Accounts"
3. Find the affected account and click "Manage"
4. Under "Restrictions," toggle any suspended selections to "Allow." If they are already set to "Allow," toggle them to "Suspended" and apply the changes.
5. Toggle any "Suspended" restrictions back to "Allow" and apply the changes.
6. The account should now be able to send mail.

If multiple accounts are affected, you can use the instructions here to suspend all email accounts on the cPanel, and unsuspend them to enforce the desired case:

How to Mass suspend or unsuspend mailboxes of a domain