title: Error 554 when using Amazon SES as Firebase custom SMTP mail server
body: <p>i'm migrating our current custom SMTP mail server for Firebase Authentication mails to Amazon SES. I generated new SMTP credentials and updated the Firebase SMTP settings:
  <a href="https://i.stack.imgur.com/zksqL.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/zksqL.png" alt="Firebase SMTP Settings" /></a></p>
  <p>But when i try to send a password reset mail, i continuously receive this error in the sender's inbox: <code>554 Transaction failed: Missing final '@domain'</code>.</p>
  <p>The same SES SMTP credentials work fine (= emails being sent), when i use them (base64 encoded) in a local CLI script (<a href="https://docs.aws.amazon.com/ses/latest/DeveloperGuide/send-email-smtp-client-command-line.html" rel="nofollow noreferrer">https://docs.aws.amazon.com/ses/latest/DeveloperGuide/send-email-smtp-client-command-line.html</a>) or on <a href="https://www.smtper.net/" rel="nofollow noreferrer">https://www.smtper.net/</a>.</p>
  <p>Additional information:</p>
  <ul>
  <li>inspired by the error message, i tried to add my maildomain to the SMTP user's name (<code>ABCDEFGHIJKLMNO@mydomain.net</code>) but received the error <code>535 Authentication Credentials Invalid</code></li>
  <li>i did not use the account's credentials (as mentioned in most topics), but the SMTP user's credentials</li>
  <li>as mentioned in other related topics, i also tried to URL encode the password with no avail (it contains a <code>/</code>). Same for base64 encoding the password</li>
  <li>the custom domain and the sender's address are verified in SES</li>
  <li>i have configured SPF and DKIM for the custom mail domain (also verified)</li>
  </ul>
  <p>If i missed any further information, please let me know, thanks in advance.</p>