Gentlemen
Shorts are not acceptable.
Wearing ties or bows is not a problem.
T-shirts are acceptable.

Ladies

Obeying Islamic rules including Hijab or Islamic dress-code is necessary in Iran. However these rules are not observed very strict, especially for tourists and foreigners. You must not worry about maintaining your hijab , since in times you have forgotten about it, the maximum penalty will be a request (usually in a kind way) to make it correct.
There are some minimum requirements for foreign women dress-code in public places
1. Color : It's a completely false belief that wearing  must be dark in Iran. There is no limitation in this respect and we recommend you make sure using light colors in summer.

2. Head : Hair should be covered. It does not mean you shall have a tight scarf around your head. Don't worry, It is very usual that some parts remain out of the cover. It's quite acceptable for women to allow whips of their hair to frame their face. Appropriate hats & caps can do this function as well as scarves. Scarf is the most common covering for head and is called "Roosari" in Farsi.

3. Body : Should be covered with loose clothes like man shirt, coat or manteau. Arms should not be bare.

4. Legs & feet : Legs should be covered down to ankles. Feets can be bare and you can wear sandals. Tight jeans are no problem.

Tip1 : In summer and hot weather, you can cheat(!) and not wear shirt under your coat/manteau. Who understands?!

Tip2 : In winter time, apart from covering the head, the rest is the same as what you wear in your home country.

 Tip3 : You better enter Iran with a coat and a scarf and select your style by watching the Iranian women in the streets.

Tip 4 : Once a chador is needed, like in holy places, it will be given upon entrance.

Tip 5 : Following the dress-code is necessary form the time of on-boarding international flights. On Iranian Flights, it is usually requested when the plane enters the Iranian boundaries.