In formal writing you capitalise words such as "ihre", "ihrem", "sie" etc. There is no big grammar behind it, it's just "nicer" that way.

If you capitalise the adresse ( ihre, ihrem,sie ) or not is totally up to you. In the past,long time ago, when i was in school, i was told to write all Pronomes , exspecially in letters ! , capitalized . Because it´s a polite way of adressing people. If you write the word non capitalized you change the grammar in a way that makes the adresse unpolite, rude and somewhat wrong. The German orthography reform of 1996 told us to forget all that and write all of pronomes small all the time. ( With the exception of "old" ways of speaking, fixed forms, like when you adresse royalty. ) Since the reform caused more confussion how to write the right way, people simply ignored it. Even worse pupils wrote things how they learned them at school and people thought them be wrong or in this case we talking about here, rude. Therefore in 2006 the reform was reformed, in the most stupid way you can do it. They simply allowed both ways ( old and new ) in each and every controversial case. Therefore today both form capitalized and not capitalized are correct. Still the reader might not be aware of this long backstory explained here , and just think you beeing rude, by not using the capitallized styl. So go for the save option , since you now allowed to do so again.

Ihre = your, and is always capitalised ihre = her, and is not captialised