I think the guy was looking for a non techy straightforward fix. This isn't it, although I know you're trying to be helpful. VM mail has been awful for years especially for Iphone/Ipad users - sometimes works, sometimes not and there is never a proper fix by either Apple or Virgin for sending mail. The only answer is to get a Gmail or 'proper' mail address.
The problem is that if you just lump all the "can receive but not send" into the same category, you miss things.
To be able to send an email a client has to do a number of things:
1. Connect to the server. 2. Exchange HELO or EHLO greetings. 3. Authenticate with the server 4. Send the sender email address (most clients will usually derive this from the From: address). 5. Send the recipient address(es). 6 Send a DATA command to indicate it's ready to send the email 7. Send the actual email data.
At ANY of the stages an email send can fail, and at step 3 a perfectly valid username and password, can fail if certain conditions fail (at last count there were around 11 reasons).
The majority of can receive, can't send issues recently have involved a VM305 error, which indicates that there's an unwanted program on their network sending spam.
HOWEVER we can't rule out another cause. For example SPAM content found errors in the OP's emails themselves.
The method outlined by @用心棒 is probably the least techy way of gathering the information we need if we're going to help the OP.
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