1. The recipient's address is wrong.
The format of the mail is :abc@moduyun.com, abc is the user name, and sohu.com is the domain name of the mail service provider. Any mistake in these two parts may lead to the rejection of the letter. For example, if a letter is written too much or too little, it may become another account or an account that does not exist at all.
2. Mass mailing was rejected.
Some free personal mailboxes are not recommended for enterprises and do not support mass mailing. Some users will send too many recipients when sending e-mail, which may cause the other service provider to suspect that spam is rejected and the letter is returned.
3. The recipient's mail is full.
If the recipient's mailbox is full, or the remaining space in another mailbox is smaller than your email, the email cannot be sent successfully.
4, send too much, the frequency is too fast
If you send e-mails to recipients too frequently, or send a large number of e-mails to users of the mail service provider, the mail service provider may temporarily refuse to receive the e-mails you send, resulting in the e-mails being returned. I suggest you try to slow down your sending speed and see if you can solve this problem.
5. The other party is forbidden to send ip.
The recipient's mail service provider blocked your email address, so you need to contact another mail service provider to unblock it.
6. The network problem timed out
If the network is unstable, the mail server of the mailbox may temporarily be unable to get in touch with the mail server of the recipient, and the mail cannot be delivered, resulting in the mail being returned.
7. The attachment is too big
The message sent was too large, which caused the server of the service provider of the recipient to reject it. It is suggested to split the mail and send it after streamlining. Usually the last number is the size of a single mail that the other party's system is allowed to receive.
Therefore, if an enterprise wants to send a large number of emails, it can register an account in Duyun, and then pass the personal/corporate authentication, so it can try out 20 emails for free.
8. The mailbox is set incorrectly.
If mailbox A is set to be automatically forwarded to mailbox B, and mailbox B is set to be automatically forwarded back to mailbox A. So your letter has been running endlessly between mailbox A and mailbox B for a long time, and the sending and receiving server is also uncomfortable and has to return it to you.