What each mail server response means
What each refusal from Mailyoto's mail servers means, quoted exactly, and who needs to act on it.
Updated 29 October 2026
When Mailyoto’s mail servers refuse an email, the server that sent it gets one of the responses below.
A code starting with 4 is temporary. The sending server tries again later.
A code starting with 5 is permanent. The sender gets a bounce.
Values in braces, such as {recipient}, stand for the real address, domain, number or link.
Email sent to your aliases#
The sender sees these in their bounce message. Refusals that happen after the message arrives also show in your logs as Failed.
| Response | Who acts, and what it means |
|---|---|
550 5.7.1 We banned {ip} from sending mail here. To appeal, write to abuse@mailyoto.com. | Sender’s provider. Mailyoto blocks the sending server’s address. |
550 5.7.1 We refuse mail from {ip} while {blocklist} lists it. Details: {link} | Sender’s provider. A public blocklist lists the sending server. See Refused by a blocklist. |
550 5.7.1 We refuse mail from {domain} while Spamhaus lists it. Details: {link} | Sender. Spamhaus lists the sender’s domain. The link opens its lookup page. |
550 5.7.25 Reverse DNS lookup failed; no valid PTR record for your IP. | Sender’s provider. The sending server’s address has no reverse DNS name. |
421 4.7.0 Too many concurrent connections; the limit is 18 at once. | Nobody. One server opened more than 18 connections at the same time. It retries. |
421 4.7.0 {ip} opened too many connections in the last 10 minutes. Retry in 10 minutes. | Nobody. One server opened more than 1,200 connections in 10 minutes. It retries, and Mailyoto accepts its email once it slows down. |
421 4.7.1 Recent connections from {ip} reached no address here. Retry in 10 minutes. | Sender. One server opened more than 150 connections in 10 minutes, and none of them reached an address here. Check the addresses it sends to. |
550 5.7.1 Our spam check scored this message {score} and refused it. | Sender. The spam check scored the message 4.5 or more. See Spam checks. |
554 5.6.1 A blocked attachment type was rejected. | Sender. The message carries a file type that Mailyoto refuses. |
550 5.6.0 A valid From: header with a single address is required. | Sender. The From line is missing, broken or holds more than 1 address. |
550 5.7.1 Delivery refused: the destination mailbox provider requires DMARC-aligned mail and this message is not aligned. | Sender. The destination refuses forwarded mail without a valid sender signature, and nothing proves the sender. See Refused for its DKIM signature. |
552 5.3.4 We take messages up to {limit} bytes, and this one declares {size}. | Sender. The sending server announced a message over the size limit. See Message too large. |
552 5.3.4 Error: Too much mail data | Sender. The message went over the size limit while it was being sent. |
555 5.5.4 We don't implement MAIL FROM's {option}. | Sender. The sending server asked for a MAIL FROM option that Mailyoto doesn’t offer. |
452 4.5.3 A report to a return path must be the only recipient. Send the other recipients separately. | Sender. A delivery report to a Mailyoto return path (rp+…) came with other recipients in the same message. |
452 4.5.3 This message has reached its recipient limit. Send the rest in another message. | Sender. The message has more than 100 recipients. |
452 4.2.2 This recipient has used this month's forwarding allowance. | You. Your account forwarded its monthly allowance. See Forwarding limit reached. |
452 4.2.2 This recipient has hit the daily inbound quota. Try again tomorrow. | You. Your account reached the most it forwards in one day. See Forwarding limit reached. |
550 5.2.1 That mailbox is disabled. | You. The account that owns the alias is locked. |
550 5.4.4 This address has no destination we can deliver to. | You. The alias has no destination left that is a valid address. |
550 5.2.0 The destination refused mail permanently, so we stopped forwarding to it. | You. Every destination of the alias refused email permanently before. See Blocked destinations. |
450 4.2.0 The destination is refusing mail for now. Try again later. | You. A destination’s mailbox was full or out of storage. Mailyoto refuses email to it for up to 72 hours, and the sending server retries. See Blocked destinations. |
450 4.7.1 This recipient is rate-limited on inbound mail. Resend later; it will go through once the limit clears. | Nobody. One server sent more email to this destination than Mailyoto passes on in a short time. It retries. |
450 4.2.1 Mail to <{recipient}> is on hold for now. Keep it queued; it goes through once the address is back on. | You. The alias is frozen after a downgrade, inside its 5-day hold. See After a downgrade. |
550 5.1.0 Mail to <{recipient}> is not deliverable at this domain. Confirm the address with the recipient before resending. | You. The alias is frozen and its 5-day hold has ended. |
Email you send through Mailyoto#
Your mail app or script sees these when it sends through smtp.mailyoto.com.
| Response | Who acts, and what it means |
|---|---|
535 5.7.8 Authentication credentials invalid | You. The username or password is wrong. The username is the full credential address. |
535 5.7.8 This SMTP credential expired at {date} {time} UTC. Extend it in the dashboard to sign in again. | You. The credential’s expiry day has passed. See When a credential expires. |
538 5.7.11 Encryption required for requested authentication mechanism | You. The app tried to sign in before starting TLS. Turn on STARTTLS on port 587. |
530 5.7.0 Sign in with AUTH before you send on this port. | You. The app sent on port 587 or 465 without signing in. Turn on authentication with the credential’s username and password. |
502 5.5.1 AUTH is on ports 465 and 587 only. | You. The app tried to sign in on port 25. Use port 587 with STARTTLS, or port 465. |
550 5.7.1 You can send only from addresses on {domain}. | You. The From address is not on the credential’s domain. |
550 5.7.1 This SMTP credential can't send as {address}. Add the address to the credential in the dashboard. | You. The credential sends only as the addresses it lists, and the From address isn’t one of them. |
550 5.7.1 We don't send mail for {domain}: it isn't on Mailyoto. | You. The sending domain is not on Mailyoto. |
550 5.7.1 We don't take mail for {domain}. | You. The app did not sign in, and the recipient is outside Mailyoto. |
550 5.2.1 The sending account is disabled. | You. The account is locked, or the domain is banned. |
550 5.7.0 This account has used this month's sending allowance. | You. The account sent its monthly allowance. It starts again at 00:00 UTC on the day your plan renews. |
452 4.2.2 This account has no sending left for today. Send again tomorrow. | You. The account sent the most it sends in one day. The Free plan sends none. A Free account always gets this response. |
550 5.7.1 We can't send mail for {domain}: we found no DKIM record for it. | You. Mailyoto can’t find the domain’s DKIM record. See Mailyoto can’t send mail for your domain yet. |
451 4.4.3 We can't send mail for {domain} yet: its DKIM lookup timed out. Try again later. | Nobody. The DNS lookup of the domain’s DKIM record timed out or failed. Your app tries again by itself. |
553 5.1.3 The recipient address is invalid. | You. A recipient address is malformed. |
550 5.2.0 The destination refused mail permanently, so we stopped forwarding to it. | You. The recipient refused your email permanently before. See Blocked destinations. |
450 4.2.0 The destination is refusing mail for now. Try again later. | You. The recipient’s mailbox was full or out of storage. Mailyoto refuses email to it for up to 72 hours. See Blocked destinations. |
550 5.7.1 Our spam check scored this message {score} and refused it. | You. Your message scored 4.5 or more in the spam check. |
Mailyoto also refuses a message with a line longer than its limit. See Line too long.
When the destination refuses#
Mailyoto accepted the email, but the inbox it forwards to refused it. The destination writes that response, and its wording differs from one provider to the next.
- A temporary refusal is tried again after 2, 5, 30, 120 and 720 minutes.
- After the last try, or after a permanent refusal, the sender gets an email titled “Message not delivered” from
no-reply@bounces.mailyoto.net. - Your logs show the destination’s response on the entry. Hard and soft bounces explains the 2 kinds.