503 Valid RCPT command must precede data

We are having intermittent sending issues with all accounts on our domain, and constant issues on one account all using Outlook.

Every message sent to an external domain returns undeliverable with the following message.

503 Valid RCPT command must precede DATA

I have tried the following:

  1. Verifying SMTP port 26 is selected instead of the default of 25.
  2. Verifying ‘My outgoing server requires authentication’ and ‘Use same settings as my incoming mail server’ are enabled
  3. Settings for POP3, POP3S (SSL), IMAP, IMAPS (SSL), SMTP, and SMTPS (SSL)
  4. Creating new profile
  5. Verifying our domain was not SPAM blacklisted
  6. Nothing has worked. I have called our service provider and they insist that the issue is on our end. They tested sending from our accounts in their building and it works fine.

I finally tested settings in Thunderbird and we are able to send externally through all accounts on all machines.

While it does work in Thunderbird, that is not an acceptable solution. Considering we have changed nothing with our account settings, I can’t see why this problem would turn up.

I’m running out of ideas at this point and would appreciate any feedback.

Apostrophes – remove any apostrophes (single quotes) or other non-alphabetic characters from your email account name(s).

VPN – if your system is trying to connect via a VPN network while you’re sending email, you may get this error. Try disconnecting the VPN before sending/ receiving mail.

Full mailbox – make sure the mailbox of the affected account is not full or has met/ exceeded its quota. You can uncheck the following option to make sure a copy of all your emails is not being left on the server (note that since the email will be downloaded to your PC after this setting change, you won’t be able to access your emails from another location if you do this). In Outlook, browse to Tools > Account Setting > More settings > Advanced > uncheck “leave a copy of this message on the server.”

SMTP port – use port 26 (instead of the default 25) for outgoing mail.

Server authentication – setup your outgoing mail to use outgoing server authentication (sends mail by logging in with your email address and password). In Outlook, browse to Tools > Account Settings > More settings > Advanced and change the outgoing server to ‘Auto.’ Next, browse back to More settings > Outgoing server and check the “My outgoing server uses the same authentication as my incoming server” box. If you select to have Outlook “remember your password” you won’t have to type it in every time you send an email.

Sender matching – make sure you’re sending from the same email address that the recipient sees your email coming in as (ie. don’t make the “from” field differ from the email address used in your outgoing mail settings).

Outlook PST files – sometimes the 503 error will occur if you have a corrupt outlook PST (mail archive) file. Try repairing the file.


The 503 valid RCPT command must precede DATA error is a common one that often occurs as a result of the spam software you’re using, or as a result of Outlook’s default spam utility. Try opening Outlook in safe mode to see if any integration with Outlook is interfering. Start > Start/Search > outlook.exe /safe (note the space between outlook.exe and /safe).

  • Pop before SMTP — Run a check for new messages (F9 key in Outlook) before sending your message and restart and rerun your spam filter application. Your IP address is entered in a relayhosts file, allowing email to be sent from your address for up to half an hour.
  • If the problem still persists check “my server requires authentication” to authenticate automatically. With this setting, you can bypass the check above and send mail without having to “pop” your mail box first. You can also try changing the outgoing port from 25 to 26, since some ISP’s block this port.
  • Some people have pointed out that it may help to remove apostrophes from your email account names (and ask senders to remove them if you’re having trouble receiving emails from them).
  • If you’re on a VPN, try disconnecting from the VPN prior to using your mail program.
  • Your mailbox may be full – try clearing out messages or asking your mail service provider to increase your mailbox limits.
  • If you are using multiple mailboxes, make sure these fixes/ settings are applied to each one of them, especially the “default” one.
  • Finally, if using a mail screening utility such as Mail Washer, you may want to disable the “automatically check at intervals” setting and check mail manually. If the system checks mail while you are sending you may get this error.

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