Error 554; SMTP only works for proton recipients

[clemtrinc]

Hello, I'm setting up IMAP and SMTP on a nextcloud instance.
Sending smtp emails works, but only when sending to proton emails.
If sending an email to another email provider, I see the email stuck in the draft box.
Using MUTT to see more details, I get :
"SMTP session failed: 554 5.0.0 Error: transaction failed: cannot send message: [2001] Message signature is missing or invalid"
Using latest version (0.2.30) of Hydroxid on FreeBSD.

[oleksulkowski]

Hi, I managed to fix this issue by simply rolling back the hydroxide version to 0.2.28. No more errors when sending mail to non-Proton email addresses.

[dehekkendekrekker]

I am experiencing this too on 0.2.30. Could this be prioritized? It's a big deal not to be able to send to other email addresses than proton's

[poksiala]

I can replicate this too

hydroxide-0.2.30_6 on FreeBSD:14:amd64 with thunderbird-146.0 . Sending to other proton mail accounts works.

[anakojm]

i am getting the same behavior where i can only send messages to protonmail users. here are my logs:

Dec 23 22:16:17 arch hydroxide[1020965]: 2025/12/23 22:16:17 $MAIL logged in
Dec 23 22:16:17 arch hydroxide[1020965]: 2025/12/23 22:16:17 creating draft message
Dec 23 22:16:18 arch hydroxide[1020965]: 2025/12/23 22:16:18 uploading message body
Dec 23 22:16:19 arch hydroxide[1020965]: 2025/12/23 22:16:19 sending message
Dec 23 22:16:19 arch hydroxide[1020965]: 2025/12/23 22:16:19 request failed: POST https://mail.proton.me/api/messages/[bunch of stuff that looks like base64]: [2001] Invalid or missing message signature, please sign out and back in again

i have, of course, signed out and back in again, several times, without success
i have also not changed any of my configurations on my side since a time when this worked fine
i can reproduce this issue with hydroxide compiled from last commit (b6a0db3)
i have more logs with ./hydroxyde -debug serve but i don’t know what contains sensitive information in there
what should be noted is that the draft message is created correctly. it also appears to me from the debug logs that the message body is uploaded correctly. this specific sending message POST to /api/messages/[same bunch of stuff that looks like base64]:

{"ID":"REDACTED","ExpirationTime":0,"Packages":[{"Type":4,"Addresses":{"non‐protonmail user":{"Type":4,"BodyKeyPacket":"","AttachmentKeyPackets":null,"Signature":0,"PasswordHint":"","Auth":null,"Token":""}},"MIMEType":"text/plain","Body":"REDACTED","BodyKey":{"Algorithm":"aes256","Key":"REDACTED"}}]}

(where REDACTED is stuff that i have redacted because i have no idea what they are and non‐protonmail user is the e‐mail address of a non‐protonmail user such as mail@example.com)
is where protonmail returns an error: &struct { protonmail.resp; Sent *protonmail.Message; Parent *protonmail.Message }{resp:protonmail.resp{Code:2001, RawAPIError:(*protonmail.RawAPIError)(0xc000391480)}, Sent:(*protonmail.Message)(nil), Parent:(*protonmail.Message)(nil)}

[emersion]

Duplicate of #182

[dnesterowicz]

The problem still occurs for me as of April 2026 with the latest version (0.2.31).

Error message: [2001] Invalid or missing message signature, please sign out and back in again
Client: aerc

Debug Log:

2026/04/10 12:26:16 << PUT /api/messages/
2026/04/10 12:26:16 &struct { protonmail.resp; Message *protonmail.Message }{resp:protonmail.resp{Code:1000, RawAPIError:(*protonmail.RawAPIError)(nil)}, Message:(*protonmail.Message)(0x8b9321ee000)}
2026/04/10 12:26:16 >> GET /api/keys
2026/04/10 12:26:16 << GET /api/keys
2026/04/10 12:26:16 &struct { protonmail.resp; *protonmail.PublicKeyResp }{resp:protonmail.resp{Code:1000, RawAPIError:(*protonmail.RawAPIError)(nil)}, PublicKeyResp:(*protonmail.PublicKeyResp)(0x8b932048c60)}
2026/04/10 12:26:16 sending message
2026/04/10 12:26:16 >> POST /api/messages/
2026/04/10 12:26:16 {}
2026/04/10 12:26:16 << POST /api/messages/
2026/04/10 12:26:16 &struct { protonmail.resp; Sent *protonmail.Message; Parent *protonmail.Message }{resp:protonmail.resp{Code:2001, RawAPIError:(*protonmail.RawAPIError)(0x8b931f18300)}, Sent:(*protonmail.Message)(nil), Parent:(*protonmail.Message)(nil)}
2026/04/10 12:26:16 request failed: POST https://mail.proton.me/api/messages/: [2001] Invalid or missing message signature, please sign out and back in again
554 5.0.0 Error: transaction failed: cannot send message: [2001] Invalid or missing message signature, please sign out and back in again
2026/04/10 12:26:20 >> GET /api/events/
2026/04/10 12:26:21 << GET /api/events/
2026/04/10 12:26:21 &struct { protonmail.resp; *protonmail.Event }{resp:protonmail.resp{Code:1000, RawAPIError:(*protonmail.RawAPIError)(nil)}, Event:(*protonmail.Event)(0x8b931f2c200)}