Code

• Everyone has an email address
• Everyone has exactly one email address
• An email address never changes
• Whenever an address does change, it’s under that user’s control
• Whenever an address does change, it’s because the user specifically requested it to happen
• Whenever an address does change, the old address will continue to work/exist
• Any one email address refers to only one single person
• Unique strings of characters all map to different addresses
• All email is hosted by a centralized system
• When email is sent to a user at a domain, it is delivered to a server whose address matches that domain
• When email is sent by a user at a domain, it is sent by a server whose address matches that domain
• All email comes from a .com, .net, .edu, or .org address
• You can filter out email based on the TLD or ccTLD from which it originates
• Having a particular ccTLD means that you prefer to receive communications in that country’s native language (for example, .fr → French)
• Email addresses only contain letters
• Email addresses only contain letters and numbers
• Email addresses only contain letters, numbers, and a handful of common punctuation marks (e.g. ., _, and -)
• Email addresses will have at least one letter in them
• An email address like ^_^@example.com or +&#@example.com is invalid
• Email is a reliable transport
• Email is an instantaneous transport
• Emails will be sent within a few minutes of their scheduling
• Emails will be sent within a few hours of their scheduling
• Emails will be sent within a few days of their scheduling
• Emails will be received soon after they’re sent
• When an email is sent it immediately goes to its destination server
• If an email bounces, the address is invalid
• If an email doesn’t bounce, the address is valid
• An address which is valid will always be valid, and an address which is invalid will always be invalid
• All email is sent via SMTP over TCP/IP port 25
• All email is sent via SMTP over TCP/IP
• All email is sent via SMTP over IP
• All email is sent via SMTP
• All email servers support the various vendor extensions by the current “everyone uses this vendor” vendor (Microsoft, Google, etc.)
• An email can only have one From: address
• The Date: header on a message is legitimate
• The Received: headers will always be no earlier than the Date: header
• All email clients support HTML attachments
• All email clients support HTML message bodies
• All email clients support MIME encoding
• Email is secure
• Encrypted email is secure
• All email is accessed via webmail
• All email is accessed via webmail or IMAP
• All email is accessed via webmail, IMAP, or POP3
• Nobody uses email anymore

See also: email is bad

Update I’m getting some good additions from folks' responses and I’ll be adding them as I see them.

From elainemorisi:

• Anyone with a .edu address is a student
• Anyone with a .edu address is a student or faculty
• Students and faculty will use their .edu address to sign up for all of their Internet accounts

Additional suggestions from a reddit thread:

• All universities provide an .edu address
• Domains will be handled uniformly between email and HTTP
• Only the owner of an address can send mail from that address
• Email attachment size will be accurate to the storage size of the attachment; relatedly, attachment filenames are the correct filenames for those file types, and that it’s safe to extract attachments to those filenames
• It is valid to remove +suffixes from email addresses (e.g. john+doe@example.com → john@example.com)
• Users actually know their own email address

From Jens Alfke:

• Email addresses are case-sensitive / can be compared by == or strcmp
• A reply to an email sent to address X will come from X (this is the mistake made by things that say “Reply with REMOVE to unsubscribe”)
• If you receive email at address X, you are capable of sending email whose From header is X.

A good one that was posted by many folks when this post went viral but seems to first be by benoliver999 on lobste.rs:

• If you send a url in an email, the user will be the first to click it

Update, 9/1/2022: Wow, I am getting overwhelmed with comments and suggestions. This really struck a nerve, huh!

I don’t think I’ll be adding any more things to this list. But here’s a few threads you can follow and post comments on:

• r/programming thread
• lobste.rs thread
• Hacker News thread
• plush.city thread

Also, I moderate every single comment that comes to this site, so please be patient if a comment you post doesn’t appear immediately.

Finally, see this followup.