Host Mx2.comcast.net[68.87.20.5] Said: 421 4.1.0 192.185.85.20 Throttled - Try Again Later

Mail to Comcast is rejected and is returned with an error message. What does this mean?

With more than 14 million high-speed Internet customers, Comcast takes its responsibility to combat spam very seriously. Comcast is aware that a conscientious residue is required to effectively reduce spam while also protecting Internet users' online experiences.

If you've received a 550 non-deliverable notification from Comcast, it will incorporate a block code and a link to i of the below FAQs which details the reason for the blocked connection and how to remediate the status.

Error Code Returned	More information
ES000001	This fault results when mail delivery is attempted from Comcast's dynamic/residential space. (Larn more)
ES000010	This error results when mail delivery is attempted from external dynamic/residential space. (Learn more)
BL000000	This error results when postal service commitment is attempted from addresses that sent mail service in patterns characteristic of spam. (Learn more than)
BL000001	This error results when mail commitment is attempted from addresses that are listed on the Spamhaus Zen Blocklist. (Learn more than)
BL000010	This error results when mail delivery is attempted from addresses that are listed on Cloudmark CSI. (Learn more)
BL000011	This fault results when post delivery is attempted from addresses that are listed on the Spamhaus Zen blocklist and Cloudmark CSI. (Learn more)
BL000100	This error results when postal service commitment is attempted from addresses that are listed on the SenderScore blocklist. (Learn more)
BL000101	This error results when mail commitment is attempted from addresses that are listed on the Spamhaus Zen blocklist and the SenderScore blocklist. (Acquire more than)
BL000110	This mistake results when postal service delivery is attempted from addresses that are listed on the SenderScore blocklist and Cloudmark CSI. (Learn more)
BL000111	This fault results when postal service commitment is attempted from addresses that are listed on the SenderScore blocklist, Cloudmark CSI and the Spamhaus Zen blocklist. (Learn more)
BL001000	This mistake results when mail delivery is attempted from addresses that are listed on the TrendMicro MAPS RBL. (Learn more than)
BL001001	This mistake results when mail delivery is attempted from addresses that are listed on the TrendMicro MAPS RBL and Spamhaus Zen blocklist. (Learn more)
BL001010	This mistake results when mail delivery is attempted from addresses that are listed on the TrendMicro MAPS RBL and Cloudmark CSI. (Learn more than)
BL001011	This error results when mail delivery is attempted from addresses that are listed on the TrendMicro MAPS RBL, Cloudmark CSI and Spamhaus Zen blocklist. (Larn more)
BL001100	This error results when mail service delivery is attempted from addresses that are listed on the TrendMicro MAPS RBL and SenderScore blocklist. (Learn more)
BL001101	This error results when mail delivery is attempted from addresses that are listed on the TrendMicro MAPS RBL, SenderScore blocklist and Spamhaus Zen blocklist. (Learn more than)
BL001110	This fault results when mail delivery is attempted from addresses that are listed on the TrendMicro MAPS RBL, SenderScore blocklist and Cloudmark CSI. (Acquire more)
BL001111	This error results when mail delivery is attempted from addresses that are listed on the TrendMicro MAPS RBL, SenderScore blocklist, Cloudmark CSI and Spamhaus Zen. (Larn more)
RL000001	This mistake results when mail delivery volumes exceed the hourly limits allowed based on the sending IPs reputation. (Learn more)

Error Code Returned	For more than information
421 - [Likewise many sessions opened]	Comcast allows 25 simultaneous connections per sending IP address. This fault results when that limit is exceeded.
421 - [Opposite DNS failure : Effort over again later]	Comcast requires all sending IP addresses have a valid rDNS. This error results when the lookup failed. This error should be treated as a temp-fail and can be retried. SERVFAIL response. 99.ix% means that the DNS somewhere up the auth tree is misconfigured or down or very badly overloaded.
421 - [Try again afterwards]	This error results for several reasons. This fault should be treated as a temp-fail and can exist retried.
452 - [Likewise many emails sent on this session]	Comcast allows g emails per session. This error results when that limit is exceeded.
452 - [Too many recipients for bulletin]	Comcast allows 100 recipients per bulletin. This error results when that limit is exceeded.
550 - [Not our customer]	This mistake results when a message is sent to a non-existent Comcast client. These errors should be treated as an "unsubscribe" by the sender, if a bulk mailer.
550 - [Your message could not be delivered due to as well many invalid recipients]	This error results when a message is sent to as well many invalid recipients. As a result none of the recipients received the message. Please check your distribution list and resend.
550 - [Invalid sender domain]	Comcast requires that all sending domains have a valid A or MX tape. This error results when neither record can be establish.
550 - [Account not available]	This error results when a message is sent to a Comcast account that is currently not bachelor.
552 - [Message size exceeded]	Comcast allows messages of 15MB or smaller. This error results when that limit is exceeded.
554 - [PTR lookup failure]	Comcast requires all sending post server IP addresses have a valid PTR record set up. This error results when the lookup failed. NXDOMAIN response. One of the authoritative servers for the relevant section of the in-addr.arpa DNS tree is proverb that in that location is no PTR record for the given IP address.

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Source: http://postmaster.comcast.net/mail-error-codes.html

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