Past perfect would be needed in that first sentence, since you are describing an event in the past that doesn't continue on to the present (the email has been sent and cannot be un-sent if it's typical email).
So your choices are:
In my last email I had almost forgotten to tell you that, so I'm going to tell you now.
In my last email I almost forgot to tell you that, so I'm going to tell you now.
Another minor issue, because if you did not tell him/her that in the email, you didn't almost forget, you did forget.
In my last email I had forgotten to tell you that, so I'm going to tell you now.
In my last email I forgot to tell you that, so I'm going to tell you now.
Past perfect is meant to describe an event that is occuring among more than one event in the past. Here, you only describe a single event (forgetting to tell in the email - note that sending the email and forgetting are not separate events because you forgot while sending the email), unless there's another past event in established context or sentences to relate it to, you should use the simple past.
Read this. Past perfect can also be used if you specify a time with a past event, but you aren't doing that either. E.g.:
In my last email I had sent at 3pm yesterday I forgot to tell you that, so I'll tell you now.