If a message you send cannot be delivered, you will receive a bounce message explaining what happened. Below are some of the things you might find in a bounce message.
Undelivered mail returned to sender from Fastmail
If you receive a bounce message back from Fastmail after sending an email, you should read it to see what it says. The message should look something like this:
This is the Postfix program at host www.fastmail.com. I'm sorry to have to inform you that the message returned below could not be delivered to one or more destinations. For further assistance, please send mail to postmaster. If you do so, please include this problem report. You can delete your own text from the message returned below.
The Postfix program
<joebloggs@somemail.com>: connect to m1.somemail.com[10.1.123.123]: Connection refused.
The final part of the message is what actually happened. The meanings of some of the most common messages are as follows:
<joebloggs@somemail.com>: connect to m1.somemail.com[10.1.123.123]: Connection refused
In this case, the other end refused our connection. This could be because the system is down, or because they are specifically blocking Fastmail. You should try sending the email again in a day or two. If it continues to fail, contact the administrator of the other system (usually postmaster@domain.tld) to find out what is happening. You will likely have to do this from a different account, as you probably won't be able to send from Fastmail to the support or webmaster address either.
<joebloggs@fastmail.com>: host xyz[/var/run/lmtpforward/outgoing] said: 550 5.1.1
<joebloggs@fastmail.com>: Recipient address rejected: User unknown in local recipient table (in reply to end of DATA command)
This error message means the user joebloggs@fastmail.com doesn't exist. You'll get this if you try to send to an email address that doesn't actually exist, or if you misspell the address.
554 <joe@blogs.net>: Recipient="" address rejected: Relay access denied
This usually means you're not using authenticated SMTP. Make sure your email client is configured correctly.
Undeliverable message from recipient's server
If the bounce message is from the recipient's server, look for a line like one of the following and read the details.
<joebloggs@somemail.com> : host m1.somemail.com[10.1.123.123] said: 550 5.1.1 <joebloggs@somemail.com>;… user unknown
When you see the message host xxx said:, it means we got an error response from the other server. The lines after this are what we got back from the other end, so you should contact the administrator of the other system if you don't understand what they mean (usually postmaster@domain.tld).
In this case, the response was "user unknown", which usually means that the email address doesn't exist on the remote system. You will also receive this message if you misspell the email address, so make sure your spelling is correct.
Send failed: invalid email address
In some rare cases, there are email addresses containing punctuation characters. To ensure they are correctly recognised by the Fastmail web interface, they need to be enclosed in angle brackets.
So weird!email@odd.org is a valid email address, but address email to this account as <weird!email@odd.org> to make it send properly.
Email addresses in this format are automatically handled by the contact list.
Characters affected are: !#$%&`*+/=?^`{|}~