This error means Apple Mail connected to your outgoing server but authentication or the connection failed partway through. In most cases it's one of three things: the SMTP port and encryption settings don't match, the saved password is stale, or the SMTP server isn't set to use the same login as incoming. Fix those in order and the send error clears.
Check your SMTP port and encryption
Flashcloud email uses mail.yourdomain.com for both incoming and outgoing. For outgoing (SMTP), the correct combination is port 465 with SSL, or port 587 with STARTTLS. If Apple Mail is set to the wrong port, or set to "Automatically detect and maintain account settings" and it guessed wrong after a network change, sending fails while receiving keeps working fine.
To check: open Mail → Settings → Accounts, select the account, go to Server Settings, and look under Outgoing Mail Server. Turn off "Automatically manage connection settings" and set the port and SSL/TLS options manually to match one of the two combinations above. Save, then send a test message.
Re-enter the password
The single most common cause of this exact error is a password mismatch between what Apple Mail has stored and what the mail server expects, usually after a password change. Apple Mail sometimes keeps trying the old password silently instead of prompting again.
In the same Server Settings pane, clear the password field under Outgoing Mail Server and type it again, even if it looks correct. Use the full email address as the username, not just the part before the @. If you're not sure what the current password is, change it from the Email Accounts tile in your hosting account, then update it here.
Confirm outgoing uses the same server as incoming
Apple Mail lets you assign a different SMTP server to an account than the one used for incoming mail, and sometimes an old or unrelated SMTP server gets left selected after adding a new account. Under Server Settings, confirm the Outgoing Mail Server dropdown points to mail.yourdomain.com for this account specifically, not a leftover entry from a different provider.
Also check that "Use only this server" is selected rather than "Use default ports," since a default fallback can route around your custom port setting.
If the account itself might be the problem
If the settings above are all correct and sending still fails, two other things are worth ruling out:
- The domain's outgoing mail authentication is broken. This usually shows up as bounces rather than a client-side send error, but it's worth a check if the problem is recipient-specific. See SPF records: what they are and how to set one up for how to verify it.
- The server or domain has landed on a blacklist, which some receiving servers reject outright. If test sends to some addresses work and others don't, check checking and delisting a blacklisted mail server.
If you're getting an actual bounce-back message rather than a client-side error dialog, decode the specific code first; see email bounce messages decoded for what 550, 554, and "relaying denied" actually mean.
When to contact support
If you've confirmed the port, password, and server assignment are correct and Apple Mail still refuses to send, open a ticket from the portal (Support → New ticket) or use live chat. A real person can check whether the mailbox is active and whether anything server-side is blocking outgoing connections from your account.