The postbox flags mean something specific
On a North American roadside postbox, a raised flag tells the carrier there is outgoing mail to collect. The emoji set encodes all four states: open and closed, flag up and flag down. 📬 closed mailbox with raised flag therefore means you have mail waiting, and 📭 open mailbox with lowered flag means empty. Almost nobody uses them this precisely, and in most of the world the convention does not exist at all, so the four are usually chosen on which drawing looks best rather than what they encode. If you want to signal an empty inbox or a full one, the distinction is there and it will be read correctly by roughly the audience that grew up with that style of postbox.
Which envelope to use for a contact line
For a bio or a contact line, 📧 is the clearest because the at sign makes it unambiguously email rather than physical post. ✉️ is the more elegant glyph, but its colour form is optional at the encoding level, so it sometimes lands as flat line art in precisely the places a contact line tends to live. 📩 envelope with arrow is better for confirming something has been sent than for inviting contact. 💌 love letter is a different register entirely and reads as affection rather than correspondence, which is why it turns up far more in Valentine posts than in anything to do with actual mail.