These are the checks that are being carried out in inbound mail, that in case of failure, will return an error to the sender, arranged according to their different inbound mail phases.
A series of reputation lists, either public or private, are looked up checking that connecting IPs do not appear on those lists. In case of being listed, mail will be rejected and a permanent error with 5XX error codes or a temporary error with 4XX error codes will be returned to the sender, pointing out which black list has listed the IP and an URL for checking the case.
The sender or the sender system's administrator, will have to check if they are indeed sending unsolicited mail, spam or malware, from that IP, and in that case stop those deliveries to be able to ask for the delisting of the affected IP. Once justified the actions taken to solve the problem, its possible delisting will be assessed.
In certain special circumstances some particular IPs may receive administrative blockages. In those cases, a permanent 5XX kind of error will be returned which will make reference to the problem occurred and to a contact to deal with the incident.
We apply rate limits to the connections per time unit to SMTP clients to limit abnormal delivery volumes to the platform.
Exceeding those limits will generate temporary rejects with 4XX codes, showing that there has been too many connections from a certain IP. The solution to the problem would require to spread in time those deliveries to avoid reaching current rate limits.
Connections from new or unknown IPs to the platform may receive temporary errors with 4XX codes. In these cases, legitimate email platforms that follow standards will keep trying delivering their pending mails.
Mail will start to be accepted from an IP once it gets to be known and no spam or malware is detected from it.
- HELO and EHLO commands are used as an introduction from the remote servers to our servers in the beginning of the message delivery. These introduced names must have a correct format according to the standard, being valid names those with a FQDN format or remote server's IP.
If the format is not correct, a permanent error with 5XX code will be returned.
To solve this kind of error, contact the source server's administrator so he can take the proper measures.
Email addresses must have a suitable format according to the standard.
According to the standards and best practices of the industry, email address' domains (e.g., example.com in firstname.lastname@example.org) must have a valid MX or A DNS resolution. In case they don't, mail will be rejected, usually with a temporary 4XX error. It is also advisable although not mandatory, that the IP to which the source domain resolves is the same domain to which resolves the connecting IP. The lack of a PTR record or the use of ramdom names could lead in certain circumstances to a temporary block.
It is not allowed that source addresses of the messages use domains that have as MX a CNAME record. The messages that do not have a correct source address for this reason, will be rejected with a 4XX code.
It will be source system administrator's task to correct the required DNS entries to solve the problem.
In certain special circumstances some email addresses or domains may receive administrative blockages. In those cases, a permanent 5XX kind of error will be returned which will make reference to the problem occurred and to a contact to deal with the incident.
We apply rate limits to source email adressess and domains per time unit to limit abnormal delivery volumes to the platform.
Exceeding those limits will generate temporary rejects with 4XX codes, showing the reason of the error.
The solution to the problem would require to spread in time those deliveries to avoid reaching current rate limits.
Deliveries to recipients whose domains do not lie on our platform are not allowed. They will generate permanent rejects which will return 5XX codes.
In case of receiving such a message, contact your administrator so he can check the configuration of your mail.
Deliveries to non-existent or invalid recipients are not allowed, even though destination domains do lie in our platform. They will generate permanent rejects which will return 5XX codes.
In these cases it must be verified that the destination address that has been written is correct, do not have any typos and that it indeed exists.
Occasionally, some destination addresses might be blocked, in case they have been the destination for malware or have been used for malicious activities.
In case so, the message would be rejected with a 5XX return code which would make reference to the problem occurred and to a contact to deal with the incident.