See the Essential parts of letters of complaint and adjustment here

Acknowledging receipt of a complaint letter

• Thank you for your letter of … [?] / [?] / in [?] with …
• I [?] to your letter of … about / [?] to …

• Thank you for [?] this matter to our attention.
• We [?] your bringing this matter to our attention.

Talking about action

• I have [?] / looked [?] / [?] … (present perfect)
• [?] checked …, it is [?] that ... / it [?] that … / it [?] appear that …

Talking about causes and effects

• This was [?] by … / was due to …
• Apparently, the [?] was the result of … / resulted from …
• The cause of / reason for the [?] was …

• As a result …
• This [?] to …
• [?] …

Apologising / reasoning

• We must [?] for …
• We [?] apologise for …
• Please accept our [?] for …
• We [?] you that …
• In future, we shall [?] to …

Denying responsibility

• I [?] to inform you that …
• I am afraid that …
• Unfortunately, I [?] point out that …
• While we can understand (your [?], etc.), we are [?] to …/ we must point out that …

Offering compensation

• As a [?] of our regret, we are prepared to …/ we are willing to …/ we would like to …
• To show [?], we will …