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:
- Verifying SMTP port 26 is selected instead of the default of 25.
- Verifying 'My outgoing server requires authentication' and 'Use same settings as my incoming mail server' are enabled
- Settings for POP3, POP3S (SSL), IMAP, IMAPS (SSL), SMTP, and SMTPS (SSL)
- Creating new profile
- Verifying our domain was not SPAM blacklisted
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.