Sendgrid based EmailRelay
Send & receive emails via an azure function. Processes the Sendgrid Inbound Parse webhook to make it work.
I recently moved from a regular app service to storage account based static websites. In the process I broke my MX records and couldn't figure out quickly how to fix them. Since I was aware of Inbound Parse I decided to throw together a small PoC, whether a "free" (as in freeloader) mail system via sendgrid is possible. After 1 hour the answer was yes; and this is the (somewhat) polished result.
- works for domains even without a mail package
- receive emails for a domain at your private email address (using Sendgrid Inbound Parse)
- send emails in the name of the domain by replying back to the domain from your private email address
See Examples for more details.
Additionally sendgrid free tier is limited to 100 mails per day (25000 mails per month if you signup via Azure) if this is not enough for you, consider a regular email service or the paid sendgrid plans.
You can also chose to only enable this system on a subdomain (e.g. foo.example.com) but then only emails of that subdomain will be received (e.g. email@example.com).
- CC/BCC are lost for relayed emails (but are visible in the blob storage if enabled)
- inline content is currently not supported (all media will be sent as regular attachments)
- Reply all/to multiple doesn't work, as each email goes through your domain, so only one recipient at a time is possible
- you can only use one email account as the sender/recipient for all emails (set via
- you can only use one domain per function (e.g. if you want to receive mails for example.com and mail.example.com you have to setup two functions)
- attachment names with non ascii characters are wrongly encoded if sent via sendgrid (the content is always correctly encoded, though)
You must first setup a Sendgrid account and connect your domain (make sure that Sendgrid is able to send emails on behalf of your domain).
You can follow their documentation to setup domain authentication.
Before you deploy the azure function, be sure to set the ResourceGroupName variables (2x) and to customize the
appSettings of the azure function deployment task in the azure-pipelines.yml file:
SendgridApiKey- key with at least
Mail Sendpermissions in your Sendgrid account
RelayTargetEmail- All emails sent to the domain will be forwarded to this email
Domain- (formats example.com and
@example.comboth work). If you have setup emails for a subdomain, then set the specific subdomain mail.example.com. Only emails matching this domain are processed
SendAsDomain- set to
trueif you want to support sending mail as the domain (see Sending mail). Defaults to false, which means only Receiving mail is possible
Prefix- customize the prefix needed to send emails. Defaults to "Relay for" (see also Sending mail)
Optionally also enable this:
ArchiveContainerName- if set all incoming emails will be stored in said container (attachments will also be extracted from the email)
Then deploy the Azure function via the azure-pipelines.yml. The pipeline will automatically create the required infrastructure and deploy the code to it.
Finally connect sendgrid inbound parse by setting your domain mx record to
mx.sendgrid.net and then providing the Azure function url (with function code) to the Sendgrid Inbound webhook.
The url will be in format <myfunction>.azurewebsites.net/api/receive?code=<code>
Once the azure function is hooked up, all you have to do is send an email to your domain.
The email should then be stored in the storage account or relayed to the target address (depending on your setup) within ~10 seconds.
Since there is no email inbox at your domain when using sendgrid, all emails will be forwarded to the configured email.
- Your domain is example.com and you expect an email at firstname.lastname@example.org (sendgrid will relay all aliases automatically)
- You have setup this function to relay emails to email@example.com
- A user firstname.lastname@example.org sends an email to email@example.com with subject
Inquiryand arbitrary content and attachments.
Based on the subject you will know who actually sent the email.
If you reply to the email (the reply will be sent to your own domain), the email is parsed again and based on the
Relay for firstname.lastname@example.org: subject prefix a new email is sent to
email@example.com with subject
Inquiry from the origin firstname.lastname@example.org.
Important: The subject prefix "Relay for: email@example.com" is used to determine whether the email needs to be forwarded to your relay target or whether it should be sent to an external email.
- The email in the "Relay for <email>: " subject is used as the actual recipient
- The email of your domain will be used as the sender (your private email won't be visible anywhere)
The prefix is parsed lax (e.g. the Re: Re: Re: spam is recognized) and kept.
Note: Only the email enteres as
RelayTargetEmail will be able to send emails in the name of the domain. If another user sends an email with such a subject, the attempt is logged in application insights and the
RelayTargetEmail will receive a copy of the email with a warning.
To send emails in the name of the domain you must send a specially crafted email to the domain.
SendAsDomain must also be set to true for this feature to work.
- Your domain is example.com and you want to send an email as firstname.lastname@example.org
- You have setup this function to relay emails to email@example.com
SendAsDomainis set to true and
Prefixis not set to a custom value (defaults to "Relay for")
- The intended recipient is firstname.lastname@example.org and the subject should be
The subject pattern is:
[RE: Fwd:...]%Prefix% %desiredRecipient%: %actualSubject%
The "Re: Fwd:" spam is automatically detected and forwarded to recipients as well.
Note that this works because email@example.com is configured as the owner and is the only one allowed to send emails in the name of the domain.
If you (or someone else) attempts to send an email in the above format to the domain (from another address) a warning will be sent to the registered owner (firstname.lastname@example.org) and no email is sent to the requested recipient.
Responding to emails
Similar to Sending mail.
- Your domain is example.com and someone sent an email to email@example.com with subject
- You have setup this function to relay emails to firstname.lastname@example.org and received said email with subject
Relay for email@example.com: Inquiry
SendAsDomainmust be set to true to allow responding in the name of the domain
You simply respond to the email (response will be sent to firstname.lastname@example.org).
Note that some email programs will change the subject to
Re: Relay for email@example.com: Inquiry (or any other known response prefix); this will be parsed correctly regardless.
Because you are sending as the configured owner (
SendAsDomain is true and the subject is in the correct format, an email is sent from firstname.lastname@example.org to email@example.com with either subject
Re: Inquiry or
Inquiry (depending on whether your email program added the
Re: or not).
If the user respons to the email again, the
Re: will be split yet again and you will receive a response email from firstname.lastname@example.org with subject
Re: Relay for email@example.com: Inquiry.