Do oclock is used by mail

A clever question!

In the context of time, "do" is indeed used in some parts of the world, particularly in the United Kingdom, Ireland, Australia, and New Zealand, to refer to the hour. For example:

• "It's half past three do" (meaning 3:30)
• "I'll meet you at quarter to five do" (meaning 4:45)

This usage is often referred to as "do" or "o'clock" and is commonly used in informal spoken language, particularly in everyday conversations.

In formal writing, such as in official documents, news articles, or academic papers, it's more common to use the 24-hour clock or the 12-hour clock with AM/PM designations, such as:

• "It's 15:30" (meaning 3:30 PM)
• "I'll meet you at 16:45" (meaning 4:45 PM)

In the context of mail, it's unlikely that "do" would be used in formal correspondence, such as letters or official documents. However, in informal emails or text messages, it's possible to see "do" used in a conversational tone, especially if the sender is from a region where this usage is common.

So, to summarize: while "do" is used in some parts of the world to refer to the hour, it's not typically used in formal writing, including mail, unless it's in an informal or conversational tone.