在使用企业邮箱时如发生相应的报错，可参考阿里云企业邮箱常见退信报错的解决方法和阿里云企业邮箱常见退信报错的解决方法进行解决。
- 421 Service not available, closing transmission channel
- 526 Authentication failure
- 524 username or passwd is NULL
- 523 cipher text is invalid base64 format
- 450 \"MAIL FROM\" mailbox unavailable
- 552 \"RCPT TO\" mailbox unavailable
- 451 Requested action aborted: local error in processing
- 504 the number of RCPT attempts is achieved the maximum threshold
- 505 the total of RCPTs is achieved the maximum threshold
- 550 mailbox is full
- 550 failed to meet SPF requirements
- 510 The length of DATA content is achieved the maximum threshold
- 510 The length of Attach is achieved the maximum threshold
- 510 The number of attachments is achieved the maximum threshold
- 551 virus infected mail rejected
- 534 Message is too long
- 521 5.2.1 \: AOL will not accept delivery of this message.
- 554 Reject by behaviour spam
- 554 Reject by content spam
- 451 Requested action aborted: local error in processing
- 400 Temporary failure
- Reject Virus Infected Mail551 virus infected mail rejected
- resolving by dns failed
- 57 This email is rejected. It contains keywords rejected by the antispam content filter
- 521 5.2.1 : AOL will not accept delivery of this message
- 451 Temporary local problem - please try later
- 550 Message is too large
- Too large/Maximum fixed size
- 552 Message size exceeds fixed limit
- 550 5.7.1 Service unavailable; Client host [x.x.x.x] blocked using Trend Micro RBL+
- Recipient address rejected: User unknown in virtual mailbox table (in reply to RCPT TO command)