“绝交信”常见的英文表达是 "a letter of severance (of friendship)" 或 "a letter ending a friendship"。
"a letter of severance (of friendship)"： “severance”有“切断、分离”之意，“a letter of severance”原指终止某种关系的正式信函，这里加上“of friendship”明确表示是终止友谊的信，即绝交信。
"a letter ending a friendship"：直白地表达了“一封结束友谊的信”，意思清晰易懂。
例句：
I received a letter of severance (of friendship) from my best friend yesterday.（我昨天收到了我最好朋友寄来的绝交信。 ）
She wrote a letter ending a friendship after that big argument.（在那次激烈的争吵之后，她写了一封绝交信。 ）